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Harry hit Cassius Warrington because Warrington had said something unforgivable about Hermione.

This was the official explanation, and it had the considerable merit of being true.

It was not, however, the whole truth. The whole truth was that Harry had spent the morning being stared at in corridors, the afternoon hearing the word Heir breathed behind his back, and the better part of supper watching the empty place where Hermione ought to have been explaining why Ron's method of buttering toast was inefficient. Her goblet had disappeared from the table. The bench had closed over the space where she sat. Hogwarts, which could grow twelve Christmas trees overnight, had tidied Hermione away in under a week.

By half past seven Harry's temper had become a cupboard into which people had been stuffing crockery all day. Warrington opened the door.

The first plate out struck him in the mouth.

Harry had not meant to punch a fourth-year. He had meant to tell him to shut up. Unfortunately, Warrington had leaned down and said, in a low confidential voice, that at least the Mudblood would be quieter as a statue. Harry's fist formed an independent government.

There followed a short, energetic period in which several things happened at once. Warrington made a noise like a wardrobe falling downstairs. Harry discovered that hitting somebody hurt the hitter as well, a point on which Dudley's career had supplied inadequate data. A cluster of Slytherins shouted. Two Gryffindors shouted more loudly, on general principles. Then somebody caught the back of Harry's robes and lifted him so efficiently that his shoes left the flagstones.

Professor Snape's voice arrived beside his ear, soft as a knife being drawn from a sleeve.

"How gratifying. Potter has at last found an academic discipline suited to his talents."

Harry, who was still attempting to reach Warrington with one foot, stopped.

Warrington was holding his bleeding lip and looking delighted. He had already discovered the first law of school fights: once a teacher appeared, the person with the blood was the victim, even if he had begun the thing and outweighed the other person by approximately one small pony. Behind him, the stone knight outside the Charms corridor continued smiling its idiotic carved smile. Hogwarts scenery was famously impartial.

"He called Hermione—"

"I heard Mr Warrington."

That was inconvenient.

Snape set Harry down. His hand remained at the nape of Harry's robes, not gripping, exactly, but making several future choices seem unwise.

"Twenty points from Slytherin," he said.

Warrington's delight fell off his face and broke.

"Professor—"

"Unless you would like to explain why Miss Granger's present condition requires your commentary?"

Warrington would not. He muttered something which might have been no, sir, if one had a charitable ear and very little self-respect.

"And fifty points from Gryffindor."

Harry found his voice. "Fifty?"

"Do you feel the blow was worth less? How modest. Detention, Potter. Eight o'clock. My office. Mr Warrington, Madam Pomfrey will repair your lip. She may be unable to do anything for the mind behind it."

Snape let go. His gaze dropped to Harry's right hand, which had begun to throb in earnest. For a moment Harry thought he would say something about it. Snape's black eyes moved over the swelling knuckles, up to Harry's face, and became unreadable.

"Do not be late."

He swept away, collecting darkness as he went.

Ron emerged from the crowd. "Twenty points from Slytherin," he said in tones of religious awe. "Did you see Warrington's face?"

"I saw it quite closely."

"You ought to go to Madam Pomfrey."

"It's only bruised."

This was true in the technical sense that no bones had yet applied in writing for separation. Harry put the hand in his pocket.

Ron looked towards the corridor down which Snape had vanished. "What d'you reckon he'll make you do?"

Harry had several ideas. None improved his evening.

At five minutes to eight, he went down to the dungeons with ten complete fingers, which seemed worth noticing while he still had them. The castle was settling for the night. Portraits muttered behind their curtains; somewhere above, Peeves sang a verse about Myrtle which would probably cause structural damage when she heard it. At the foot of the dungeon stairs, warmth and supper smells gave way to wet stone and the mineral chill of the lake.

Snape's office door opened before Harry knocked.

"If you intend to loiter theatrically, Potter, select a corridor in which I am not working."

Harry went in.

The office was warmer than the classroom and smelled of bitter herbs, hot metal and the damp-stone odour all dungeons cultivated in the belief that it gave them character. Jars crowded the shelves. Some of their contents floated. Others had sunk and were plainly waiting for nightfall. One held a pale flower whose petals followed Harry as he crossed the room. On the central worktable stood baskets of roots, bunches of leaves, a stone mortar, three knives and enough glass bottles to alarm a milkman.

There were no cauldrons to scrub.

Harry distrusted this at once.

Snape was at the far bench writing labels in a narrow, vicious hand. He did not look up.

"Wash your hands."

Harry obeyed at the little basin in the corner.

"With soap, Potter. Even Gryffindors are generally introduced to the concept before adolescence."

Harry used the soap. It smelled of rosemary and something metallic. The water turned pink over his knuckles.

"Hold out your hand."

"It's fine."

Snape looked up.

Harry held it out.

Snape took his wrist and turned the hand once beneath the green-shaded lamp. His fingers were cool and dry. "You have split the skin over the second knuckle and bruised the third metacarpal."

"I said it was bruised."

"And this triumph of diagnosis has naturally rendered treatment unnecessary."

He fetched a brown bottle, dabbed three drops across the cuts and released Harry before Harry had quite prepared himself to be released. The sting vanished. The swelling did not, but settled into a dull pulse.

"You will use the left hand for anything requiring force."

"I'm not left-handed."

"Then the evening may offer you the rare pleasure of acquiring a skill."

Snape indicated the worktable. "You will prepare ingredients for Monday's classes. If you damage anything through haste, carelessness or the irresistible urge to display your famous flair, you will replace it from your Gringotts vault."

Harry stared at the baskets. "All of it?"

"No, Potter. The castle employs six hundred house-elves solely to admire that table. Begin with the valerian. Separate sound roots from moulded, remove the root hairs, rinse, and slice to an eighth of an inch."

Snape returned to his labels.

Harry sat. He selected the paring knife rather than the broad chopping blade, braced a knobbled root with his left hand and began.

The knife was beautifully sharp. Aunt Petunia's knives were not. She said sharp knives were dangerous, by which she meant Dudley might touch one; Harry was expected to make do with blades whose edges had the moral resolution of cold porridge. He adjusted after the first slice, using less pressure. The valerian gave off a warm, old-sock smell.

He sorted as he worked. Firm roots in one tray, spongy or blackened ones in the waste bowl. Root hairs came away under the back of the knife. The cleaned pieces went into water only briefly, because roots left soaking took up water and went soft. Harry stacked two at a time where their thickness allowed it and cut the rounds even.

After several minutes he became aware that Snape had stopped writing.

"What?" Harry said.

"An eloquent question."

"What am I doing wrong?"

Snape came to the table. He picked up three slices and aligned their edges. "Nothing. Yet. Why did you discard this?"

He pointed to a root in the waste bowl.

"Mould."

"Where?"

Harry turned it over and showed him the grey bloom tucked into a crease below the crown.

Snape's nostrils narrowed. This was generally a preliminary to unpleasantness, though Harry could not see how mould had offended him personally.

"And this?"

"It's gone soft."

"The outer tissue is soft. The core is usable."

"Right."

"You will trim it, not consign seventeen Sickles' worth of medicinal root to the bin."

Harry retrieved it. "You said separate sound roots from moulded."

"I had not realised the instruction required a legal definition of root."

"Well, it does if the outside doesn't count."

There was a pause. Harry recognised it too late as a place where a sensible boy would have stopped.

Snape leaned one hand on the table. "The mould penetrates along damaged vascular channels. Surface softness from cold storage does not. Remove the affected tissue until the colour and resistance are uniform. If the centre smells sour, discard it."

Harry cut away the soft layer. The centre was pale and crisp.

"Like cutting the bad bit out of a potato," he said.

"If the comparison assists you."

It plainly did not assist Snape, who had the air of a man forced to discuss lacework with a troll.

Harry finished the valerian. Snape inspected the tray, said, "Adequate," and set down a bundle of broad silver-green leaves.

"Bruise the moonwort. Do not tear it."

Harry looked over the tools, selected the flat of the broad knife, and pressed each leaf once, firmly enough to darken the veins without breaking the surface.

Snape's hand closed over his wrist.

Harry went still.

It happened before he could help it. One instant he was at the worktable; the next his shoulders had locked, his breath had gone shallow and every part of him was waiting to learn what the hand meant. Snape let go at once.

"Where," he said, more quietly, "did you learn to do that?"

Harry looked at the leaf. "Do what?"

"Bruise it."

"You told me to."

"Most second-years interpret bruise as an invitation to pulverise."

"That'd be crushing."

"Yes."

"And you said bruise."

Snape's expression suggested Harry was being insolent in a new and administratively difficult fashion.

"Who taught you the distinction?"

"No one. It's just—" Harry moved a shoulder. "Cooking. You bruise mint if you want the oil out without bits everywhere. Aunt Petunia hates bits."

"You cook."

This sounded less like a question than an accusation of secret Morris dancing.

"Everyone cooks," Harry said, because at Privet Drive everybody who mattered had always spoken as if the work simply happened. Breakfast appeared; shirts became ironed; pans returned clean to cupboards. The person doing it was no more relevant than the hot-water tank.

"No," said Snape. "They do not."

Harry thought of Ron, who had once attempted to slice a sausage by prodding it with a fork until it surrendered. "Oh."

"How often?"

"Dunno. Breakfast. Dinner sometimes."

"For your family."

Harry snorted. "They weren't going to eat what I made for myself."

Snape's face altered by such a small amount that Harry might have imagined it. The line beside his mouth deepened; that was all.

"How old were you when you began?"

"I don't know. Seven? Maybe six."

"Six."

"A fry-up isn't hard. You only have to get everything done together. Bacon first if Uncle Vernon wants it crisp, sausages before the eggs, mushrooms in the bacon fat unless Aunt Petunia's dieting, toast last or he complains it's cold."

Snape looked at the laid-out tools, the waste bowl on Harry's right, the cleaned roots protected beneath damp cloth and the unwashed moonwort kept well away from both.

"Indeed," he said.

He crossed to the shelves and returned with another bundle.

"These are sopophorous beans. You will not cut them."

"Bruise them?"

"Crush them."

"Why?"

Snape went very still.

Harry wondered whether nobody had previously tried asking why in detention, or whether they had and their remains were in the jars.

"Because," Snape said at last, "the juice is held in pressurised vesicles beneath a resilient magical membrane. A blade opens the bean along one plane. Crushing ruptures the membrane throughout and releases the volatile fraction before it can retreat into the fibre."

Harry considered the bean. "Like crushing garlic with the flat of a knife."

"Garlic does not actively attempt to retain its essence."

"You haven't met Aunt Petunia's garlic."

Something moved at the corner of Snape's mouth. Snape did not smile, so Harry put it down to a muscle spasm brought on by proximity to Gryffindors.

"Use the silver press," he said. "And Potter—"

Harry looked up.

"Ask before deciding that a magical plant resembles something from your aunt's kitchen. Some ingredients object."

Harry's first detention ended at half past ten, by which point he had prepared enough ingredients to poison, cure or moderately inconvenience most of the school and had learnt that sopophorous beans, unlike garlic, bore grudges.

Snape inspected everything. He found two valerian slices too thick, one moonwort leaf torn at the stem and a sopophorous bean whose juice Harry had allowed to touch the iron rim of the waste bowl. He missed nothing. Harry began to suspect that Snape's eyes could rotate independently, though he had no proper name for the suspicion.

"Return tomorrow," Snape said.

"Tomorrow? You said detention. Singular."

"And then you questioned a direct instruction, struck a metal bowl with a silver press, and placed aconite within six inches of hellebore. Your plural has been earned."

"You never said they couldn't go near each other."

"It is common knowledge."

"Not to me."

Harry said it sharply. He was tired, his hand ached, and the word common had begun to feel like a stone people threw at him and then blamed him for failing to catch.

Snape regarded him across the worktable.

"Evidently," he said.

Harry waited for the usual list: lazy, arrogant, inattentive. None came.

"Tomorrow. Seven. Bring your Potions text."

The following evening Snape set three objects on the table: a pewter cup, a copper cup and an iron cup. Into each he poured water from the same jug. Harry's textbook lay open beside them, weighted at one corner by a bezoar and at the other by Snape's contempt.

"Touch them."

Harry touched the pewter. Cool. The copper was cooler. The iron made the pads of his fingers prickle.

"The water's the same temperature."

"Thermally, yes."

"There's another kind?"

Snape shut his eyes briefly.

"Open your book to page twelve."

Page twelve showed a cauldron, a warning not to inhale fumes, and a cheerful drawing of a boy wearing dragon-hide gloves. Harry had always thought the boy looked smug for someone about to lose his eyebrows.

"Find the definition of magical temperature."

Harry scanned the page. Then the next. "There isn't one."

"Page thirty-eight."

There was no definition there either. There was an instruction to lower the flame if a Cure for Boils emitted red sparks.

"Page seventy-one."

Harry found a note in the margin: Cold-natured ingredients should be warmed magically before introduction.

"It doesn't say how."

Snape took the book from him. He turned pages with increasing contempt, as if the author were present in miniature and hiding between them.

"This text was approved for use by children," he said.

"I'm a child."

"So I am repeatedly informed."

Snape placed the book aside. "Thermal heat is molecular motion. Magical heat is the tendency of a substance to release, absorb or provoke magical activity. They may correspond. They need not. Ashwinder shell is thermally inert once shed and magically hot for days. Powdered moonstone may be warm from a mortar and remain magically cold."

"How can you tell?"

Snape tapped the iron cup. "You already did."

Harry touched it again. The prickling climbed into his knuckles. "That's magical heat?"

"Magical activity. Your nerves interpret it according to affinity, sensitivity and expectation. Another person might describe pressure, cold, or a taste of metal. This is why a competent brewer uses instruments and does not depend upon poetic fingers."

He drew his wand. A thin silver ring appeared above the water, trembled over pewter, widened over copper, and flattened against iron.

"Pewter buffers magical change. Copper conducts it. Iron grounds it. Hogwarts requires pewter because pewter is patient with fools."

"And you use copper."

Snape glanced towards the row of gleaming cauldrons behind his desk. "I am not obliged to be patient with my own work."

This was so exactly Snape that Harry nearly laughed.

For the next hour they made no potion. Snape made Harry sort ingredients into groups, which seemed a poor use of magic until the groups began refusing to stay simple.

"Dittany," Snape said.

"Active. It heals."

"In essence. Honey."

"Carrier?"

"Binder in emulsions, carrier in syrups, stabiliser in several respiratory draughts, contaminant in most antivenoms."

"That's cheating."

"The ingredient is unlikely to apologise for its complexity."

"Bezoar."

"Active against poisons."

"And grounding agent in certain metallic reactions."

"You said one category."

"I said classify them. You assumed singularity."

Harry scowled at the bezoar. It looked like something a cat would be ashamed to produce.

"This is common sense too, is it?"

Snape's hand paused over a jar of powdered pearl.

"No," he said. "This is first-year theory."

"We didn't do it."

"You were assigned chapters two through five."

"Chapters two through five said pearl distributes moon-aspected suspensions. It didn't say what a carrier was. Hermione knew, but Hermione knows everything."

"Miss Granger knows what she has read. That is not the same thing, though the distinction has thus far evaded her."

Harry would have liked to hear Hermione's response to this. It would have been alphabetical.

Snape pulled Harry's textbook back and read several pages in silence. "What did you believe a stabiliser did?"

"Stopped it exploding."

"Sometimes."

"And a binder?"

"Made it thicker."

"You confused magical binding with viscosity."

"Nobody said there were two sorts."

"You have watched me distinguish them."

"You've watched me fail to distinguish them and taken points off."

The dungeon became very quiet.

Harry wished his fist would form another independent government and punch him instead.

Snape laid the book down with care. "Give me an example from cooking."

"Of what?"

"A binder."

"Egg in meatballs. Flour in sauce, sort of."

"A carrier."

"Oil. Water. Stock."

"A catalyst."

Harry hesitated. "Yeast? No, that becomes part of it. Heat, maybe."

"Heat supplies activation energy. It is not generally classed as an ingredient."

"Baking soda and vinegar?"

"Both are consumed. Think of an agent which assists a process without providing the principal substance or effect."

"A bit of salt on onions makes them give up their water faster."

"Incorrect. Salt changes the movement of water through the onion and remains in the result. It assists, but it is not a catalyst."

"Then I don't know."

"Fresh pineapple tenderises meat because an enzyme accelerates the breakdown of protein. The pineapple is not the catalyst, Potter. The bromelain within it is."

Snape reorganised the jars. This time he explained each class with a mundane parallel first and its magical use second. It was irritating how quickly the jumble began to make sense. A potion had bones. Some ingredients did the thing; some told the thing where to go; some prevented the thing from quarrelling with itself on the journey. Recipes had ceased to look like commandments and begun to look like very cramped maps.

Harry had always thought brewing was a list of instructions guarded by Snape. Add six porcupine quills. Stir four times. Heat until pink. If it was orange, lose points. If it was green, run. The recipe knew what it meant, Snape knew what it meant, and Harry's part in the arrangement was to be wrong where they could both see him.

Now, very reluctantly, it began to resemble cooking.

Not in the ingredients. Aunt Petunia had strong views on newts.

In the reasons.

"So when the book says simmer until violet," Harry said slowly, "violet isn't what you're trying to make it."

Snape looked up sharply. "Continue."

"It's how you know it's got there. Like onions going clear. You don't cook them for five minutes because five is magic. You cook them until they're soft, and five minutes is how long the writer reckoned yours would take."

There was an odd silence.

"Translucent," said Snape.

"What?"

"Onions become translucent. Not clear."

"You knew what I meant."

"Precision is not an ornamental virtue, Potter. Violet indicates that the infusion has reached the appropriate magical saturation. Time is an approximation dependent upon flame, cauldron, ambient field and ingredient strength."

"Then why don't the instructions say that?"

Snape looked at the cheerful boy in dragon-hide gloves.

"A question," he said silkily, "I shall raise with the author, should misfortune ever place him within wand range."

The next evening became another detention, and the evening after that followed it with the inevitability of a tax bill. By Thursday, Harry's detention had become three detentions, which seemed mathematically unfair even allowing for Snape.

He had not told Ron much. Ron believed Snape was making him disembowel things and Harry saw no reason to spoil a comforting theory. He certainly did not say that on Wednesday Snape had shown him how clockwise stirring laid a magical current into a liquid, while an anticlockwise turn cut across it and caused two properties to meet. He did not say that the explanation had kept him awake, picturing currents folding invisibly inside every cauldron in the classroom.

It was bad enough being thought the Heir of Slytherin. Harry did not need a reputation for enjoying homework.

On Thursday, a cauldron waited on the worktable.

It was small, old and made of dull pewter. Beside it stood mooncalf milk in a blue bottle, dried dittany root, honey, rosemary, ginger, powdered pearl and a fragment of ashwinder shell under glass.

"What are we making?"

"You are making nothing. You are observing."

"That's not a potion."

"Your powers of identification continue to dazzle. It is Hestia's Restorative, an eighteenth-century domestic preparation for magical depletion."

"Domestic?"

"Made in homes, Potter. A practice once common among families whose children did not require a professor to define common."

Harry ignored that. "What's it do?"

"Restores a modest amount of magical energy, produces peripheral warmth and encourages natural sleep. It is not a substitute for food, proper rest or treatment of an underlying condition."

"So it's soup."

"It is not soup."

"For magic."

"No."

"Warm, restorative, helps you sleep—"

"If you refer to it as soup again, you may observe from the corridor."

Harry sat on the stool.

Snape prepared the bench with a neatness that made Aunt Petunia's kitchen look struck by weather. Ingredients stood in order, but not recipe order. The dittany rested beneath damp linen. Powdered pearl waited farthest from the window. Ginger remained whole.

"Why's that last?"

"Why is what last?"

"The ginger. You put things you need first nearest your hand, except that."

"Fresh ginger oxidises when cut. Its magical warmth degrades with its volatile oils."

"So you cut it just before it goes in."

Snape gave him a look which, from anybody else, might have meant yes.

He warmed the mooncalf milk over a low blue flame. "This batch was collected from a dam three nights after the full moon."

"Does that matter?"

"Mooncalf milk is most stable during the waning phase. Milk from the waxing phase promotes magical growth and is unsuitable for a restorative intended to settle the body's field."

"The bottle only says mooncalf milk."

Snape turned the label. On its reverse, tiny writing listed the date, phase, animal, pasture and the name of someone called E. Crumb.

"There is no such ingredient as simply mooncalf milk. Provenance matters whenever a material carries the history of the organism which produced it."

"Does everything?"

"No. Limestone is not improved by an account of its childhood."

Harry watched a skin form at the edge of the milk. "It's catching."

"It is not."

"There."

Snape bent over the cauldron. A faint crescent of cream clung to the side, no wider than a thread.

"The flame is correct," he said.

"Then it's too hot."

"The flame is correct. The potion is magically overheated."

"Those aren't the same thing."

"We have discussed this."

"We discussed cups. Cups don't catch."

Snape swept his wand in a tight counterclockwise circle over the cauldron. The crescent dissolved.

"The ashwinder shell is releasing stored magical heat through the carrier faster than the pewter can buffer it. Thermal reduction would slow the dittany extraction and produce a weak restorative. The magical excess must be grounded independently."

"With the wand."

"With controlled wandwork. Do not wave yours over an active potion until you can distinguish extraction from evacuation."

Harry tucked his wand farther into his robes.

Snape added bruised rosemary, then shaved dittany root so thinly that light passed through it. Each slice curled on the milk and disappeared. The smell changed from sweet to green and medicinal.

"Listen," Snape said.

Harry listened. The simmer made an uneven sound: three soft taps, a pause, then two.

"What am I listening for?"

"If I tell you, you will hear it whether it exists or not."

They stood in silence. Three taps, pause, two. Three, pause, two. Then three, pause, three.

"It changed."

Snape added a thread of honey. "The carrier has reached saturation."

The pearl went in next. The potion turned opaque, then cleared from the centre outward in a silver ring. Snape stirred twice clockwise and held the spoon still. The liquid continued rotating after the spoon stopped.

"Magical current," Harry said.

"Yes."

It was the first unqualified yes Snape had given him. Harry felt absurdly pleased and immediately distrusted the feeling.

Snape peeled the ginger with the edge of a spoon.

"That's wasteful," Harry said.

"It removes the skin without cutting into the flesh."

"No, I mean putting it in raw."

Snape's eyes lifted. "Enlighten me."

"If you toast it first, it'll taste warmer. Not hotter-hot. More—gingery."

"A technical term of rare distinction."

"You know what I mean. Dry pan, just until the cut side browns. It changes the oil."

"It changes the physical oil. The magical property sought here is gentle, sustained warmth. Direct heat risks converting it to a rapid stimulant."

"Risks, or does?"

Harry knew at once he had gone too far. Snape's face became blank in that dangerous way it did before points vanished in flocks.

Then Snape looked at the ginger.

"The household literature states that it does."

"Have you tried?"

"No."

"Then how do you know?"

"I possess the uncommon capacity to learn from written scholarship."

"The same scholarship that doesn't define magical temperature?"

There are moments when life opens two roads before a person. One road is broad, safe and labelled KEEP YOUR MOUTH SHUT. Harry had never been much of a traveller upon it.

Snape stared at him for five long seconds.

"Fetch the copper test cauldrons," he said.

They divided the base.

Snape sliced half the ginger raw. Harry toasted the other half in a little silver pan, because iron would ground the warmth and copper might feed it. Snape made him state this before allowing the flame. The ginger hissed faintly. Its cut face coloured gold, then brown at the edges, and the dungeon filled with a warm sharp smell.

"Now," Snape said.

Harry took it off.

"I was going to."

"You were going to wait for smoke."

"I wasn't."

He might have been. The evidence had not yet matured.

They added equal weights to the two cauldrons. The control brightened to soft amber. Harry's flashed once, subsided, and developed narrow lines of light that ran through it like cracks in ice.

"Did I ruin it?"

Snape did not answer. He passed his wand over the first cauldron, then the second. Silver figures assembled in the air: temperature, field intensity, rate of decay. Harry's batch showed a sharp early rise and a steeper decline.

"No," Snape said finally. "You altered it."

"For better?"

"Faster onset. Greater peripheral warming. Approximately half the duration and a mild stimulatory effect which may interfere with sleep. Different is not a childish synonym for better."

Harry looked at the lines. "Could be useful if somebody was cold and needed to wake up."

"Possibly. Or dangerous if administered after certain magical shocks."

Snape opened a black notebook and pushed it across the table.

"Record precisely what you did. Weight, surface colour, pan material, flame height, duration and sensory changes."

"You want me to write in your book?"

"Unless your observations have learned to preserve themselves."

The pages were full of Snape's experiments. Tiny cauldrons numbered in rows. Ingredient lots. Lunar phases. Temperatures. Marginal notes such as false stability at 11m—surface calm, active lower current and supplier claims naturally shed; fibres show cutting damage. One page recorded twelve failed attempts to keep powdered pearl suspended in a dreamless-sleep base. The twelfth had leaked through the paper in a violet star. Snape had written around it.

Failures had not been crossed out. They had been dissected.

Harry looked from the notebook to the shelves, and the office changed shape around him. The bottles were not a collection. They were evidence. The three copper cauldrons were not spares; they were comparisons. Snape did not merely know Potions. He asked them questions and refused to be lied to.

"Potter. The ginger will not remember its own weight."

Harry bent over the book.

His handwriting looked round and untidy beneath Snape's, but Snape did not take the notebook away.

The experiment ought to have ended when the figures stopped changing. Instead, Snape washed both test cauldrons and made Harry run the trial himself. Harry cut one sample too thickly, toasted the other unevenly and let the milk develop a skin while trying to manage both flames. Snape refused to rescue either batch. By the time Harry had skimmed the milk and strained the scorched ginger, he understood that two tasks were not the problem; he had arranged the bench for Snape's hands instead of his own. He moved the knife, scale, waste bowl and pan handle, and the work immediately became possible. Snape ended the trial before tiredness could invent anything valued chiefly by undertakers.

"Why keep those?"

"Because the skin formation may have trapped a portion of the ashwinder heat. Failure is not absence of information."

"Even if it's my fault?"

"Especially then. Ingredients are more consistent than people."

Harry put on his cloak. At the door he looked back at the rows of retained mistakes, each dated and numbered.

"Professor?"

"What now?"

"Did you make all of those?"

Snape's stare could have stripped paint.

"No, Potter. They crept in from the corridor to damage my reputation."

Harry went upstairs smiling, which he explained to himself as exhaustion.

The weekend gave Harry just enough time to discover that the currents from Snape's laboratory had followed him upstairs. He saw them in tea when Ron stirred in sugar, in bathwater circling the plug, and once, distractingly, in the gravy at Sunday dinner. By Monday morning he was almost eager to try the idea in class.

Monday's Potions lesson began badly and improved only in the sense that nobody died.

The class was brewing a mild Deflating Draught. Hermione's empty stool stood beside Harry like a missing tooth. Ron, who became more reckless in exact proportion to the number of sensible people absent, had forgotten to clean their cauldron after the previous lesson.

"It's only a bit of dried nettle," he whispered.

"Nettle's an active in swelling mixtures."

Ron stared. "Since when do you talk like that?"

"Since—shut up and scour it."

Across the aisle, Draco Malfoy had heard. "Potter's been reading," he announced. "Someone alert the Prophet."

Harry kept his head down. Malfoy had been especially unbearable since the duelling club, when Harry had spoken to the snake and the school had decided this was more suspicious than the actual enormous message daubed on a wall.

Snape wrote the instructions on the board.

They were different.

Under slice six shrivelfigs thinly, he had added: controlled exposure of binding sap; do not crush seeds. Under heat to turquoise, there was: colour indicates release of the fig's constrictive property into the carrier. The explanations were short, cramped and squeezed into the available space as though Snape begrudged them the chalk.

Hermione would have copied them before breathing. Harry simply stared.

Snape turned. His gaze travelled over the room, caught on Harry for less than a second and moved on.

"If the novelty of written instruction has overwhelmed you, Potter, I can provide a picture."

"No, sir."

"Then begin."

Harry organised the bench. Shrivelfigs left, daisy roots right, leech juice at the far corner so Ron's sleeve could not knock it over. He checked the root before cutting. Ron watched him with the wary interest of a dog observing a card trick.

"You always put things in whatever order they fit," Harry said. "Don't."

"They all end up in the cauldron."

"So does your spoon. Doesn't make it an ingredient."

Ron moved the spoon.

Their first stage went well. Harry sliced rather than chopped. He watched the potion instead of the clock and lowered the flame when the turquoise began at the edges, because their battered pewter cauldron heated unevenly. Snape passed behind them.

"Your heat is too high."

"The flame's already low."

"Did I say thermal heat?"

Harry looked at the cauldron again. The liquid was turning with a current of its own, too quickly, and the powdered daisy root had begun to gather in the centre. "The shrivelfig's releasing too fast. So... magically hot?"

"Eventually."

"One anticlockwise stir. Shallow."

Harry did it. The gathering root dispersed.

Malfoy dropped his knife.

Ron looked from Harry to Snape. "What," he breathed, "was that?"

"A stir."

"He said precisely. To you."

"Maybe he's ill."

At the next table Neville's potion gave a wet cough.

Snape crossed the room in two strides and extinguished the flame. "Do not touch it."

Neville froze with one hand above the cauldron.

The potion looked perfect: clear turquoise, gently steaming.

Harry smelled something sweet beneath the shrivelfig. Sweet and metallic, like a spoon left in jam. The hairs rose on his arms.

"Sir," he said.

Snape already had his wand out. A translucent shell dropped over the cauldron just as the potion inverted. Turquoise became a flat, lightless black. The cauldron did not explode. It sighed.

Every brass button on Neville's robes shrank to the size of pinheads.

"A false equilibrium," Snape said to the class. "Mr Longbottom introduced the leech juice before the fig binder had dispersed. The potion appeared stable because the incompatible properties had separated into upper and lower currents."

Neville was white. "I'm sorry, sir."

"An explosion would have been preferable."

Several people looked unconvinced.

"An explosion has the courtesy to announce that something has gone wrong. Bottle a falsely stable potion and the failure travels to the patient."

Snape vanished the black liquid, then looked at Harry. "What did you detect?"

Every face turned.

"It smelled wrong."

"A description suited to a three-year-old."

"Sweet. But metal underneath. Like—like old pennies in syrup. And it made my arms prickle."

"Copper and overripe fruit," Snape corrected. "The odour marks inversion of the leech extract. The prickling is magical feedback from a saturated carrier. Five points to Gryffindor."

Silence struck the room like weather.

Snape's expression suggested that anyone commenting would be used to test Neville's next batch.

After class, Ron cornered Harry outside the dungeon.

"What are you doing in those detentions?"

"Ingredient prep."

"You knew what was wrong with Neville's potion."

"It smelled funny."

"Snape gave you points."

"It was probably an accounting error."

Ron narrowed his eyes. Harry knew that look. Ron used it when deciding whether a chess piece was bait.

"You're enjoying it."

"I'm not."

It was only partly a lie. Harry disliked being mocked, corrected, kept in a dungeon until ten or made to define words which Snape could plainly define better himself. Every answer sprouted three new questions. Snape could make the word ladle sound like an accusation of moral weakness.

He enjoyed the moment a potion stopped being arbitrary: the instant a colour became evidence, a smell became warning, and the apparently demented instruction on the board turned out to have machinery behind it.

That was different.

He went to the hospital wing after lunch. Madam Pomfrey allowed him ten minutes beside Hermione and shooed him from the bed when he attempted an eleventh. Hermione lay stiff and cold-looking, her bushy hair spread over the pillow. Harry told her about the five points because it was the sort of impossible news which ought to wake anyone.

She remained stone.

"You'd like the new instructions," he said. "They say why."

His voice sounded foolish in the bright ward. He stopped.

On the way out he passed Warrington, whose lip had been repaired so perfectly that Harry's knuckles resented the waste of effort. Warrington looked at Harry's hand, then at the matron's half-open door. His smirk faltered, and he decided to keep walking. Apparently medicine could repair a lip without removing the memory of how it had happened.

Harry put both hands in his pockets and kept walking.

This was called progress. It felt overrated.

That evening, with Warrington's repaired mouth and Hermione's unmoving face still crowding his thoughts, Harry returned to the dungeons. Snape put a wooden spoon into his hand and made him practise laying currents through water and powdered moonstone: clockwise to establish the flow, anticlockwise to cut it, and a figure eight to fold two currents together without forcing their contact.

On the seventh clockwise turn, the moonstone formed a smooth silver spiral. When Harry reversed, the spoon struck an invisible resistance. The crossed currents flashed, and the powder dropped to the bottom.

They repeated the pattern in pewter and copper. Pewter let the resistance build gradually; copper snapped the current against the spoon so quickly that Harry lost it.

"Copper doesn't make it stronger," he said. "It makes it faster."

"More responsive. Strength depends upon what is being conducted and whether the geometry reinforces it."

"Like shouting in a tunnel?"

"Crude, but serviceable. A tunnel may amplify one frequency and swallow another. Copper faithfully transmits both wisdom and stupidity."

"And pewter?"

"Muffles each. This is why the school board's periodic campaign for cheaper aluminium cauldrons is idiotic. Aluminium carries thermal heat unevenly under magic and reflects several common binding currents."

"Why do they want it, then?"

"It is cheap. Governing bodies possess a mystical reverence for savings whose cost will be paid by someone else."

Harry grinned. "Uncle Vernon would like them."

"Your uncle's judgement is not a standard I had considered. It does explain much about the proposal."

Snape demonstrated the figure eight with barely a movement of his wrist. Harry's first loops came out lopsided, but on the fifth attempt both held for three seconds.

"Better," Snape said, which was not the same as good and was therefore credible.

"Why isn't any of this in the book?"

"It is assumed that children observe household brewing before school."

"Ron hasn't."

"Mr Weasley has watched his mother stir clockwise to disperse and fold to bind. He may not know the terminology, but the movements are familiar. Mr Malfoy has almost certainly been instructed by a family tutor. Miss Granger reads beyond the assigned text until she encounters an explanation."

"And me?"

Snape cleaned the spoon with one stroke of his wand. "You were expected to infer a magical practice you had never seen from instructions written by people who had seen it all their lives."

Harry waited for the insult. Snape's voice held none, which made the sentence feel unstable.

"That's stupid," Harry said.

"Yes."

"You thought I knew."

Snape placed the spoon down. "Yes."

It fell well short of an apology. Snape did not appear built for those; some internal part would probably reject one like an incompatible transplant. Still, he met Harry's eyes.

"Did you know all this before Hogwarts?"

"No."

Harry blinked. He had expected Snape to emerge from infancy criticising the magical temperature of his bottle.

"My mother brewed household remedies. I observed technique. Theory I learned here and elsewhere. Much of what is called instinct is merely forgotten instruction."

"But you're brilliant at it."

The words escaped without permission.

Snape looked almost offended. "Do not become maudlin."

"I wasn't."

"You were approaching it at speed."

He replaced the water with a thin infusion of knotgrass and set a brass instrument above it. Tiny arms marked the direction and tension of the magical flow.

"Again," he said.

This time, when Harry's left-hand loop collapsed, Snape did not seize the spoon. He tapped the brass dial and said, "Your pressure changes at the crossing."

Harry adjusted it himself.

Harry's figure-eights remained uneven, but over the next two evenings they stopped collapsing altogether. Two days later, while the brass instrument was still set out on the bench, a parcel of fluxweed arrived from the Hogwarts greenhouses. The label read south bed, full moon, first frost, cut 23:47–00:12, P. Sprout.

Harry read it twice. "Why does twenty-five minutes matter?"

"The moon reached opposition at eleven fifty-nine. Samples cut before and after differ slightly in transformational bias."

"How?"

Snape selected two stems. "Before opposition, the plant's magic is accumulating. It favours becoming. After, it favours the maintenance of a changed state."

"It's the same full moon."

"To the unaided eye. Astronomy is not decorative stargazing, despite Professor Binns's valiant efforts to make every subject appear dead."

They sliced equal weights into two crystal dishes. The early-cut fluxweed curled towards a drop of salamander blood; the late-cut curled around it and held.

Harry bent closer. "So one starts the change and one makes it stay."

"Broadly. The effect is modest, but in Polyjuice a modest bias compounded across a month may determine whether the transformation settles cleanly."

"So ingredients cut before the full moon help start changes, and ones cut after make them last."

"Fluxweed may do so near opposition. If you apply one plant's response to every ingredient in the storeroom, you will eventually produce either nonsense or an evacuation."

"Right. Just fluxweed."

"At present, just these two samples of fluxweed. General principles are earned through repeated evidence, not enthusiasm."

Harry labelled the dishes pre-opposition and post-opposition. His lettering leaned uphill.

"Thursday," Snape said, "you will prepare a suspension using each. We will measure duration."

"Thursday isn't detention."

"It will be if you continue pointing that out."

Another Thursday followed, and then enough Tuesdays and Thursdays that Harry ceased counting them as detention at all. Snape, however, kept exact accounts. One Tuesday, as Harry corked jars of sliced asphodel, Snape ended the official punishment with all the warmth of an eviction notice.

"Your debt to school discipline is discharged," he said. "You will therefore cease appearing at my office and distracting me from productive work."

Harry set down the cork. "Right."

He had known the detentions would end. That was a property of detention, like pain and Filch. Still, the room seemed to close its door before he had left it.

Snape labelled a jar.

"The Hestia trial requires a second observation at forty-eight hours. Thursday, seven o'clock."

"That's not detention?"

"No."

"Then what is it?"

Snape looked at him with exhausted dislike. "Research, Potter. Try not to cheapen the word."

Harry went back on Thursday.

He went the next Tuesday too, because the toasted ginger batch had developed a faint sediment and Snape said observations made by somebody else did not count as Harry's. After that there was a trial comparing rosemary bruised with stone, silver and wood. Then a batch of honey from hives kept beside a magical orchard behaved differently from ordinary heather honey, and Snape made Harry inspect both suppliers' records.

Nobody called these meetings detentions. Nobody called them lessons either.

Snape remained Snape. He deducted points from Harry on Monday for allowing his lacewing flies to scorch, then made him repeat the extraction on Tuesday until he could hear the change in bubbling. He called Harry's notes "a massacre of the written language" and supplied him with a ruler for tables. When Harry forgot that powdered bicorn horn carried both an inherent strengthening property and an associative duality, Snape asked whether the fight had dislodged part of his brain.

Harry began looking forward to Tuesdays.

He did not examine this closely. Harry's inner life was like the cupboard under the stairs: there were things in it best left behind boxes.

One evening in December, while they were testing a variant fever reducer, the voice came through the wall.

Hungry... so long... kill...

Harry's hand stopped above the cauldron.

The diced hellebore dropped all at once.

The potion flared white.

Snape knocked Harry's hand aside, seized a jar of iron filings and threw in a pinch. The flare collapsed with a sound like breath sucked through teeth.

"Idiot! What were you—"

Rip... tear... blood...

Harry turned towards the wall.

"Potter."

The voice moved overhead. Harry heard it pass through stone, horribly pleased with itself.

"It's going towards the stairs."

"What is?"

Harry was already at the door.

Snape caught his shoulder. Harry twisted free.

"The voice!"

He ran.

Behind him Snape shouted his name, but Harry knew the voice was real. He knew where voices like that led. Mrs Norris. Colin. Justin. Hermione. If he was quick enough this time, perhaps he could get there before somebody became another still figure beneath a sheet.

He raced up the dungeon stairs, along an empty corridor and through the entrance hall. The voice faded near the marble staircase.

There was nobody there.

Snape arrived without appearing to have hurried, a skill teachers practised for the humiliation of children.

"Explain."

"It was in the wall."

"What was?"

Harry heard how it sounded.

Snape's face had become the face from class: hostile, certain, shut. Harry remembered the duelling club. Parseltongue, the whispers had said. Heir.

"Nothing."

"You abandoned an active cauldron and ran through the castle because of nothing."

"I thought I heard someone. I was wrong."

"You turned before the sound entered the corridor."

Harry's scar prickled, though Voldemort was nowhere about. He folded his arms.

"Perhaps fame improves the hearing."

Snape's mouth thinned. "Return to the laboratory."

The fever reducer was ruined. White strings had formed through the liquid where magical heat had denatured the hellebore. Harry stood at the bench while Snape disposed of it in silence.

"I'm sorry," Harry said.

"Are you?"

"Yes."

"For the potion?"

Harry said nothing.

Snape placed the ladle in the sink. "You are very still."

"What?"

"In class, when accused of something you have done, you argue. When accused of something you have not done, you become offensive. When embarrassed, you stare at the floor."

"I don't."

"You are presently doing all three and none."

Harry looked down. Since that proved Snape's point, he jerked his head up again.

"I hear a voice," he said. "Sometimes. In the walls."

Snape did not speak.

"It says it wants to kill people. I heard it before Mrs Norris, and before Justin and Nearly Headless Nick. I heard it the night Hermione was—" His voice caught. He cleared it roughly. "Nobody else can."

"What language?"

Harry laughed once. "You know."

"Parseltongue."

"Yes."

"And you elected not to mention that an unknown serpent is travelling within the castle walls."

"To who? Everyone already thinks it's me. Lockhart thinks I want attention. McGonagall gets that line between her eyebrows and tells me not to wander alone. Dumbledore asked if there was anything I wished to tell him like he already knew the answer and was waiting to see if I'd lie."

"Did you?"

"Did I what?"

"Lie."

Harry glared.

"Yes," Snape said. "There you are. Offensive."

He began cleaning the copper cauldron with a cloth dipped in vinegar solution.

"Describe the voice."

"I just did."

"No. You reported meaning. Describe the sensory event."

Harry tried. "It isn't loud. But it feels close even when it's far away. Sort of behind my teeth."

Snape's hand stopped.

"Continue."

"There's pressure in my ears. And a smell, except not really. Damp. Stone after rain. Sometimes metal."

"Before or after the speech?"

"Before. I think."

"Does your scar react?"

"No."

"Temperature?"

"Cold. Not proper cold. Magical cold, maybe."

Snape folded the cloth precisely. "You have described a magical trace transmitted through a conductive structure."

"The pipes."

"Possibly. The castle's water system contains lead, copper and iron sections of several ages, all carrying centuries of residual enchantment. If a sufficiently powerful creature moves through or adjacent to it, the network may conduct a magical signature. Parseltongue permits you to interpret the vocal component."

Harry stared. "You believe me?"

"Your account is testable. Belief is for claims made by people too lazy to devise tests."

It was not yes. Harry trusted it more than yes.

Snape took three stoppered vials from a drawer and a pair of dragon-hide gloves.

"Where did you hear it most clearly tonight?"

Ten minutes later they were climbing out of the dungeons, Snape carrying the stoppered vials and Harry trying to match the voice's route to the pipes he could see. They found residue in a disused washroom off the second-floor corridor.

It lay invisibly along the base of a cracked copper pipe. Snape revealed it with a diagnostic charm: a thin green-black sheen, like oil seen in moonlight.

"Don't touch it," Harry said.

Snape's head turned by degrees. "Your concern for my continued existence is overwhelming."

"You tell me that every time anything's interesting."

"Because your definition of interesting includes bodily fluids, unknown venom and cursed handwriting."

The last item was a guess. Harry hoped his face did not admit this. Snape was examining the pipe and missed it—or stored it for later, as Snape stored most unpleasant things.

Using silver forceps, he drew a thread of residue into a vial. It recoiled from the silver and clung to the glass.

"Serpentine?" Harry asked.

"The affinity is reptilian. Beyond that, conclusions would be premature."

"It wants to kill people."

"An attitude hardly exclusive to reptiles."

Back in the laboratory, Snape established six tests in crystal dishes. Crystal provided no magical buffer, which made it dangerous for brewing and useful for observation. Into the first sample he introduced powdered limestone. Nothing happened. Iron filings made the second draw inward. Dittany blackened on contact with the third.

"Venom," Harry said.

"Or a corrosive magical secretion."

"That's venom with extra words."

"A useful description of scholarship, according to many students."

Snape placed a pinhead of residue into a neutral suspension. The clear liquid turned yellow, green, then a deep bruised purple. A ring of frost formed around the dish though the room was warm.

"Magically cold," Harry said.

"Predatory suppression. It dampens the field of living tissue before physical injury."

Harry thought of Mrs Norris rigid on the torch bracket; of Colin's round face behind his camera. "Can it petrify?"

"The residue alone is insufficient."

"But the creature?"

Snape's expression sharpened, not at Harry but at the problem. "Possibly."

He tested age by adding two drops of a solution that smelled of dust and apples. Silver filaments rose from the sample, multiplied, and broke at different lengths.

"What does that mean?"

"The enchantment binding the trace to the pipe is extremely old. Centuries, not decades."

"The Chamber was built by Slytherin."

"That is a legend."

"The voice is real."

"One verified claim does not convert every tale attached to it into fact."

Harry paced to the end of the bench. "So what now?"

"Now I report a dangerous serpentine magical trace in the plumbing to the Headmaster. You do not seek the creature, enter pipes, interrogate suspicious sinks or otherwise demonstrate the survival instincts of a concussed Flobberworm."

"I don't do things like that."

Snape looked at him.

"Often," Harry amended.

"You will also provide dates, locations and exact words heard."

"Why?"

"Movement pattern. Hunting intervals. Whether its language distinguishes sight from smell, hunger from command, prey from target."

Harry fetched the ruler and opened his notebook.

They worked past midnight.

At some point Snape put a mug beside Harry's elbow. The tea was strong, barely sweet and contained milk. Harry drank it without comment. Snape did not ask how he took tea. Adults were always knowing little things they had no reason to know. Harry had found it safest not to encourage them.

At half past twelve, Harry yawned over the word vicinity and blotted it into viciously, which was probably more accurate.

"Enough," Snape said.

"There are two more attacks to map."

"They will remain attacked until morning."

He took the quill from Harry's hand. Harry's fingers closed on it automatically.

For a second neither released it.

"Potter."

Harry let go.

Snape's eyes dropped to his hand, then to the empty mug and the hollows beneath Harry's eyes. "When did you last sleep?"

"Last night."

"For how long?"

"Enough."

"An exact measurement found in no responsible text."

Harry shrugged. The voice had been worse in dreams. Hermione never moved. The corridor stretched. Something slid behind the walls while Harry arrived a minute too late, over and over.

"I sleep."

"You also eat, I suppose."

"Of course I eat."

The answer came too quickly. Snape heard it; Harry could tell. But the professor only put the tea things aside.

"Hospital wing."

"I'm not ill."

"No. You are exhausted, magically depleted and vibrating from three cups of tea. Madam Pomfrey would skin me if I returned you to your dormitory in this condition."

"You made the tea."

"A lapse in judgement for which I am already suffering."

Instead of the hospital wing, Snape made half a dose of Hestia's Restorative.

Not the toasted-ginger variant. Harry noticed.

"Slowly," Snape said, giving him the cup. "If you gulp it, the binder may engage unevenly and your hands will go numb."

It tasted of honey, herbs and the warmth left in a kitchen after baking. Magic loosened behind Harry's ribs. His toes grew warm.

"It's soup," he said.

Snape took the cup away. "Out."

Harry was asleep before he reached Gryffindor Tower and had to be rescued from a suit of armour by Percy at two in the morning.

By breakfast, the Headmaster had closed the second-floor corridor. Professors examined the pipes while students were redirected around them, and rumour concluded before lunch that a colony of flesh-eating slugs had invaded the girls' lavatories. Fred and George encouraged this by selling salt charms.

No one told the students about the residue.

No further attack came before Christmas.

The castle settled into snow, holly and the tense cheerfulness of people determined to celebrate while something hunted them. Suits of armour wore sprigs of holly. Enchanted snow fell warm and dry in the Great Hall. Every burst of laughter sounded a little too loud, as though the school meant to convince the thing in the walls that nobody was frightened.

Most students went home. Harry stayed, as did Ron. Snape remained because Snape appeared to have been built into the dungeons and could not be removed without damaging the foundations.

On Christmas Eve, Harry found a parcel at the foot of his bed. It contained dragon-hide work gloves in his size, a proper paring knife with a horn handle, and a copy of Elementary Principles of Material Affinity.

There was no card.

"Hermione," Ron said at once.

"Hermione's Petrified."

"Could've ordered it before."

The book's margins contained narrow corrections in black ink. On page nineteen, beside a confident assertion that copper amplified all potion effects, someone had written: Conducts. Amplification is contingent upon current geometry. Appalling.

Harry closed the book.

"Yeah," he said. "Maybe."

He used the knife after Christmas dinner to shave chocolate from a block in the kitchens. The house-elves provided flour, butter and an alarming number of opinions. Harry made biscuits because Ron had never had the sort with melted chocolate in the middle and this represented a gap in his education which no responsible friend could permit.

He made too many.

Harry often cooked too much when nobody was shouting about cost. He put six in a napkin and carried them to the dungeons.

Snape opened the door. He wore shirtsleeves and held a glass pipette. Without robes he looked narrower, less like a bat and more like a man who had forgotten several meals while disapproving of them.

"Potter. Has the castle caught fire?"

"No."

"Has Weasley?"

"Not today."

"Then why are you here?"

Harry held out the napkin. This had seemed less stupid upstairs.

"Biscuits."

Snape regarded them as if Harry had offered six dead mice and expected applause.

"I can see what they are."

"I made too many."

"A failure of arithmetic."

"Do you want them or not?"

Snape took the parcel. "Your knife technique remains uneven when cutting chocolate."

"You haven't seen it."

"There are fragments in the fold of the napkin."

Harry hated him.

On Tuesday, the empty napkin had been washed, folded and placed beside Harry's stool.

They resumed work on Hestia's Restorative. The sediment in Harry's toasted-ginger variation had worsened. Under magnification it proved to be pearl carrying denatured ginger oil out of suspension.

"So it failed," Harry said.

"It separated. The primary effect remains, but dosage would vary between the upper and lower portions. Unacceptable for administration. Useful as information."

"Could honey hold it?"

"More binder would slow onset, eliminating the only possible advantage."

"Different honey?"

Snape set down the lens.

Harry went to the shelves and brought back the orchard honey and the heather honey. "The orchard one bound the rosemary test faster."

"Because it carries ambient magic gathered from the fruit blossoms. It may also interact with the ginger's warmth."

"We could test it."

"We could," Snape said. "After you tell me why the jar labelled orchard honey is an inadequate experimental description."

Harry turned it around. "Apiary, date, plants in range, weather, storage, hive enchantments."

"And?"

"Whether it was heated before bottling."

"And?"

Harry squinted at the supplier's crabbed writing. "The queen's age?"

"Irrelevant to this property."

"You said the animal's history matters."

"When the property derives from the animal. Here the bees are carriers. The significant provenance belongs to the forage and handling."

"That's not fair."

"Magic is under no obligation to construct a syllabus for you."

They tested three honeys. Orchard honey held the toasted oil but fizzed alarmingly with dittany. Heather honey failed to bind. Dark forest honey formed a stable suspension, but converted the rapid warmth into a pulsing effect which made Snape use a word Harry had previously heard only from Uncle Vernon during tax returns. By evening, all three variants were failures, and Harry's head buzzed with the next possibilities: change the grinding surface, infuse the ginger into the honey first, use less pearl, or toast the root whole.

"Professor?"

"No."

"I haven't asked."

"You intend to propose a fourth trial. It is nine o'clock."

"Tomorrow, then."

"No."

"Thursday?"

"Potter."

"We haven't tried—"

"You have a school timetable, an endangered friend, and approximately seven years of sleep to recover. Research will survive forty-eight hours without you."

Harry began cleaning the glass dishes. "I sleep fine."

"You slept in a suit of armour."

"That was the potion."

"The potion encourages sleep. It does not court medieval metalwork."

Harry scrubbed a stubborn crescent of honey. "I can come tomorrow."

"You cannot."

"Why not?"

"Because I said so."

There it was: the wall adults erected when reasons grew inconvenient. Harry dropped the brush into the sink.

"Fine. It's your experiment."

"Yes."

"Your book, your ingredients, your dungeon. You don't need me."

"No."

Harry's face burned. Of course Snape did not need him. Snape could cut ginger, hear saturation and write straight without a ruler. Harry had been useful because he knew one thing about toast.

He reached for his bag.

"You mistake me," Snape said.

Harry kept packing.

"I do not need you here tomorrow. That is not synonymous with never requiring your observations again."

"Sounds the same."

"Only to someone determined to misunderstand."

Harry shoved the book into his bag. Snape seized the strap.

Harry jerked back.

The bag opened. Two wrapped rolls, an apple and three biscuits fell onto the floor.

Neither spoke.

Harry picked them up quickly. "They're mine."

"I did not say otherwise."

"I was saving them."

"For?"

"Later."

Snape looked at the food, then at Harry. "Dinner ended two hours ago."

"I ate."

"Did you?"

"Yes."

Harry had eaten. He had taken one helping because taking seconds attracted attention, and put the rolls away because food available now was not proof of food available later. This was simple household management. Aunt Petunia would have understood, though she would have called it stealing.

"Potter," Snape said, "how frequently do your relatives withhold meals?"

Harry's stomach dropped.

"They don't."

"Your answer is contradicted by your face, your pulse and the bread in your schoolbag."

"It's not a big thing. If I haven't done chores, or Uncle Vernon's angry, I miss one. Sometimes."

"Define sometimes."

"I don't count."

"You count ingredient slices."

"That's different."

"Is it?"

Harry put the food back. "I'm going."

Snape released the strap.

"Tomorrow," he said, "you will sleep. Thursday, you will return at seven."

Harry waited.

"And you will eat before you come."

"I always eat."

Snape's gaze was black and level. "We have established the limited value of that statement."

Harry reached the door before Snape spoke again.

"How many people do you cook for?"

Harry turned. "What?"

"At your relatives' home. Breakfast. Dinner. How many?"

"Four."

"Including yourself?"

"Usually."

The word had got out before Harry could stop it. Snape's attention fixed upon it.

"Define usually."

"It depends if there's enough. Dudley eats a lot."

"Your cousin's appetite determines whether you are permitted food you prepared."

"It's not like that. Aunt Petunia buys it."

"And you cook it."

"That's one chore. Everyone has chores."

"Do they? What does your cousin cook?"

Harry pictured Dudley holding a frying pan. The pan did not survive the picture.

"He does other things."

"Name one."

"He's—" Harry searched. Dudley carried his own schoolbag on days when Piers was absent. He sometimes moved his plate nearer the sink, though this was usually because it stood between him and pudding. "He's younger."

"By how much?"

"A month."

Snape became extremely quiet.

"It isn't a problem," Harry said. "I'm good at it."

"Those statements are unrelated."

"If I can do it, why shouldn't I?"

Snape's gaze moved to the neat ranks of jars Harry had prepared. "Being useful does not entitle them to use you up."

Harry nearly laughed. At Privet Drive, capability was the only thing which created obligation. Once Aunt Petunia discovered he could reach the cooker knobs, breakfast became his. Once he could carry the hoover, stairs became his. Once he knew not to mix whites and colours, all laundry containing anything embarrassing became his fault.

"You make students prepare ingredients."

"Students attend this school to learn Potions. They do not exist to maintain my household."

"I lived there."

"So do your aunt, uncle and cousin."

"You don't understand."

"No," Snape said. "I am beginning to."

That was worse. Harry opened the door.

"Potter."

"I'm tired. You said I had to sleep."

"Thursday at seven."

Harry left without answering.

He did return on Thursday. He spent the whole afternoon deciding not to, which naturally made punctual arrival unavoidable.

There was food on the office table: thick vegetable soup, bread, cheese and two apples. Actual soup, which Harry felt strengthened his position regarding Hestia's Restorative.

"Sit," Snape said.

"I'm not hungry."

His stomach growled with the loyalty of a paid witness.

Snape served himself a bowl. This was cunning. If he had simply ordered Harry to eat, Harry could have refused on principle. Eating in front of him converted refusal into sitting at a table while someone else had supper, which was both miserable and silly.

Harry sat.

Snape cut bread. "The soup is not poisoned."

"You'd say that if it was."

"If I wished to poison you, I would select a method less insulting to my profession."

Harry took a bowl. He ate slowly at first and then, despite intentions, quickly. Snape made no comment when he took more bread.

"Did you make this?" Harry asked.

"Yes."

"It's good."

Snape looked as uncomfortable as he had over the biscuits. "It is soup. The standard is not exacting."

"You chopped the carrots too small."

"Their dimensions are intentional."

"They're mushy."

"You may leave."

Harry ate another spoonful. "Still good."

For a while the only sounds were spoons and the low ticking of cooling glass. Then Snape said, "Your Head of House is responsible for your welfare at school. The Headmaster is responsible for your placement outside it."

Harry stopped eating.

"No."

"I have not asked a question."

"I'm not going to live somewhere else."

"Why?"

"Because I can't. There's protection. My mother's blood. Dumbledore said Privet Drive keeps me safe."

Snape's spoon touched the bowl without sound. "He explained this to you?"

"Sort of."

"Define sort of."

"He hasn't. I heard things."

"From whom?"

Harry thought of overheard conversations, Hagrid's half-answers, the way adults exchanged looks above his head as if he were furniture with unruly hair.

"People."

"An unimpeachable source."

"It's true."

"It may be. Safety from one threat does not excuse exposure to another."

"They're not dangerous."

Snape's eyes went to the food still bulging slightly in Harry's bag.

"I'm not going to get them in trouble."

"Why not?"

The question was so plainly ridiculous Harry had no answer prepared. You did not get the Dursleys in trouble. They were the people who defined trouble. If a teacher complained, Aunt Petunia smiled and said Harry told stories. If neighbours noticed, Uncle Vernon explained that difficult boys needed firm handling. If Harry told too much, summer remained waiting at the end of June.

"It'll make it worse," he said.

Snape did not say what will? He understood at once, which was unhelpful of him.

"Retaliation."

Harry bent his bread into smaller pieces. "You don't know them."

"No. But I know people who depend upon privacy for their courage."

The words had edges. Harry looked up. Snape's face gave nothing away, but one hand had tightened beside his bowl.

"I'm fine there."

"You sleep in a bedroom?"

"Yes."

"With a door you may open?"

Harry felt the trap close. "Now."

Snape's hand flattened.

"Previously?"

"It was only last summer. Uncle Vernon was stopping me doing magic."

"How?"

"Lock. Bars. A cat flap for meals."

Snape stood so abruptly the chair struck stone.

Harry flinched. Soup moved in the bowls.

Snape saw. He became motionless.

"My anger is not directed at you," he said, each word placed separately. "Do try to keep up."

Harry had never heard an adult say that while looking so furious.

"I didn't do magic. Dobby did. They thought—"

"Stop parroting their excuses."

"I'm explaining."

"You are reciting their case for them. I assure you, they will have ample opportunity to do that themselves."

Harry shoved back his chair. "You don't get to say that. You hated me until a month ago."

The words landed. Snape did not avoid them.

"Yes," he said.

Harry's anger lost its footing.

"I mistook ignorance for idleness because it suited me to do so," Snape said. Each word seemed to cost him a small coin. "An error I do not intend to repeat."

It was an apology after all, though built in the Snape fashion: severe, inconvenient and entirely without cushions.

Harry looked at the floor.

"Professor McGonagall will be informed," Snape continued. "Not tonight. Not without a plan to prevent retaliation. You will not be sent home with a cheerful letter and no protection."

"Dumbledore—"

"Will also be informed. He will explain the wards in words suitable for the person expected to live behind them."

"And if he says I have to go back?"

Snape gathered the bowls. "Then he will discover that I am capable of being exceptionally tedious."

Harry had seen Snape reduce grown shopkeepers to nervous discounts over improperly dried wormwood. He found this more reassuring than he ought to have done.

At the sink, Snape said, "The toasted ginger trial has stabilised."

Harry looked towards the samples. "It has?"

"The whole-root toast with forest honey. The onset remains faster, duration is reduced by only twenty-two per cent, and sediment is negligible."

"So it worked."

"It produced a stable variant with potential limited application."

"That's what I said."

"It is very much not what you said."

Harry carried his empty bowl to the sink. Snape accepted it without remark.

"Can we test it?"

"After you finish the apple."

"I'm full."

"Potter."

Harry took the apple.

Snape opened the black notebook to a fresh page. At the top he wrote Hestia's Restorative, rapid-onset variant: Potter preparation.

Harry stared at the line until the letters threatened to become ordinary again.

"You put my name."

"Whose preparation was it?"

"Mine."

"Then resist the urge to make a ceremony of accuracy."

Harry bit into the apple to hide his face.

Two evenings later, Harry sat in the Headmaster's office between Professor McGonagall and Snape while Dumbledore explained the blood wards. The silver instruments on their spindle-legged tables clicked and puffed as if this were ordinary school business. Harry disliked them for it.

"Your mother's protection is anchored by residence with her blood family," Dumbledore said. "For the present, Privet Drive remains the strongest defence against Voldemort and those who served him."

"Strongest against one threat," Snape said. "Not all threats."

McGonagall's mouth had become a line sharp enough to mark parchment. On the desk lay a list in her handwriting: unlocked bedroom door, removal of the window bars, access to Hedwig, permission to contact Hogwarts. Harry read it twice and felt the summer taking shape around the words.

"If you send them a letter, they'll know I told," he said. "They'll behave until whoever checks has gone, and then it will be worse."

"Then we shall not rely upon cheerful correspondence," McGonagall said. "There will be unannounced inspections and a direct Floo connection to my office."

"And meals," Snape said.

Dumbledore added, "Reasonable access to meals."

Snape took the parchment, crossed out the phrase, and wrote three meals daily, nutritionally adequate, without conditions. He returned the quill to its stand with the expression of a man who had corrected a particularly dangerous essay.

"That is fussy," Harry said, because the alternative was admitting that the new sentence made his throat hurt.

"So is starvation," Snape replied.

Dumbledore studied the amended list, then signed beneath it. For the first time, the wards sounded less like a reason Harry had to endure Privet Drive and more like a problem the adults were required to solve.

Winter loosened slowly after that. Snow slid from the greenhouse roofs. The lake stopped pressing black ice against the dungeon windows, and Professor Sprout began appearing at meals with dragon-dung under her fingernails and an expression of guarded triumph. Harry's Tuesdays and Thursdays settled into the school week as firmly as Quidditch practice. Some evenings he and Snape brewed. On others they did nothing more exciting than compare two lots of dried dittany while Snape complained about suppliers and Harry learnt that merchantable quality was an expression meaning not quite fraudulent enough to prosecute.

The new arrangements did not remove Privet Drive, but they gave summer doors, witnesses and consequences it had never possessed before.

Harry said this was fussy. Snape said so was starvation.

Nothing between them became easy. Snape could still flay Harry's mistakes in class and then spend an hour after supper teaching him why they were mistakes. Harry still concealed things by instinct and resented being caught. They quarrelled about cauldron alloys, ingredient substitutions and whether Harry had eaten. At least now they generally knew which quarrel they were having.

In May the mature Mandrakes were finally cut and the restorative was brewed. Hermione woke with the solution clenched in one rigid hand and a fury at having missed months of lessons.

The creature was a basilisk. The pipes were its road. The entrance was in a girls' lavatory, which Ron considered typical of dark wizards because nobody sensible would have looked there willingly.

For less than a day, the answer seemed simple enough to hand to the professors. Harry had begun to believe that this was how disasters could be stopped: notice the wrong thing, tell someone competent, and let the adults close around it. Then Ginny Weasley vanished, the message on the wall changed, and the school convulsed into motion. The professors scattered to search and seal the corridors. By the time Harry and Ron understood where Ginny had been taken, there was no competent adult within reach. They went to Lockhart because they had not yet learnt that consulting an adult and consulting a competent adult were separate operations.

This was precisely the sort of behaviour Snape had prohibited. Harry remembered the prohibition as the stone door sealed behind him, and the Chamber answered with the damp, metallic pressure he had first felt through the walls.

The basilisk's voice no longer travelled through pipes. It pressed directly behind Harry's teeth, vast and hungry. Magical cold gathered along the channels cut into the floor before the serpent emerged, the residue announcing its route a heartbeat before the stone began to tremble.

Harry moved on that warning. Tom Riddle shouted a command; the cold surged from Harry's left, and he threw himself aside before the basilisk's head struck the place where he had been. Fawkes came in fire and music, the Sorting Hat fell into Harry's hands, and the Chamber abandoned any remaining interest in orderly investigation.

Once Fawkes blinded it, the basilisk thrashed beyond any pattern Harry could follow by sight. He listened instead: scales against stone, the change in its hiss, the magical pressure flowing ahead of its body like current through copper. When the cold rose beneath him, he drove the sword upward.

A fang entered his arm. Venom burned along the wound with the terrible clarity of an ingredient that knew exactly what it was meant to do. Then Fawkes's tears struck the puncture, and two opposed properties met inside him. For one sickening moment Harry felt both working. The phoenix's magic held.

Across the Chamber, Ginny's magic still ran towards the diary. Harry saw the pattern now: the book was carrier, binder and active agent together, sustaining the memory while drawing substance from its victim. He drove the basilisk fang through the cover. Ink bled across the floor; the current broke; Ginny breathed. The diary did not.

When Harry woke in the hospital wing, Snape was standing at the foot of his bed. Dawn had turned the high windows grey. The ward smelled of dittany, clean linen and the peculiar burnt-feather scent Fawkes had left behind. Harry's mouth tasted of metal. Every beat of his heart tugged at the new skin inside his elbow.

Professor Dumbledore and Mr Weasley were speaking beside Ginny. Madam Pomfrey moved between screens. Ron was asleep in a chair with his mouth open.

Snape said nothing.

Harry considered pretending to be unconscious. His body, which had faced Tom Riddle with more enthusiasm, betrayed him by opening both eyes.

"You entered the pipes," Snape said.

"Technically it was a chute."

"You interrogated a suspicious sink."

"Hermione worked that bit out."

"You sought the creature."

"Ginny was dying."

Snape's hands closed once around the bedrail. They were perfectly steady before and after.

"Yes," he said. "She was."

Harry looked away. The hospital windows were dark. His arm felt heavy where basilisk venom had opened it, healed it and left a shining pink scar.

"The fang went through the diary," he said. "The ink came out like blood. I think it was a personal agent. The writing knew who it was because part of him was in it."

Snape went still.

"Not now, Potter."

"But that's how it worked, isn't it? The diary was a carrier and a binder and—"

"Not now."

Harry shut his mouth.

Snape drew the chair closer and sat. Adults did not sit by Harry's bed unless they wanted answers or the chair happened to be there. Snape appeared to want neither. He inspected the curtains with deep dislike.

"Did the residue tests help?" Harry asked.

"They established the creature's route and allowed the staff to ward several junctions. It altered its hunting pattern."

"So not enough."

"Enough is a childish unit of measurement. Evidence answers the question it is capable of answering."

"If we'd found the entrance sooner—"

"You would still have attempted to confront it."

Harry could not deny this with Snape looking at him.

"You were right about the pipes," Snape said.

"Hermione was."

"You reported the trace. Miss Granger identified the creature. Mr Weasley located the victim. The phoenix blinded it. The Hat supplied a sword. You killed it. Must heroism consume every available portion, or can you permit others a helping?"

Harry thought about this. "The Hat didn't do much."

Snape's eyes closed.

Madam Pomfrey approached, saw Snape, and altered course with the brisk purpose of a warship.

"Five minutes, Severus. He needs rest."

"He has spent the year resisting it. I doubt five minutes will reverse the habit."

She pointed towards the door. Snape rose.

"Professor?"

He looked back.

"Am I still rubbish at Potions?"

It was not what Harry had meant to ask. He had meant to ask whether Snape was angry, whether he had believed Harry, whether Thursday at seven still existed in a world where diaries talked and phoenixes wept and people could nearly die between supper and curfew.

Snape considered him with the severity of an examiner.

"Yes."

Harry's chest sank.

"At present," Snape added. "Fortunately, incompetence is considerably easier to remedy than stupidity."

He reached the curtains.

"And you are not stupid, Potter. Reckless, secretive, insubordinate and possessed of the self-preservation of a moth in a candle shop, certainly. But not stupid."

The curtains fell behind him.

Harry lay awake for several minutes, which was difficult, because the hospital wing had gone blurry. He took off his glasses to clean them and found, unreasonably, that the room remained blurred.

He blamed the phoenix.

Harry left the hospital wing two days later. Examinations had been cancelled, which Ron considered proof that basilisk-slaying ought to be added to the curriculum, but classes continued until the train home. At the final Potions lesson of the year, Snape wrote a Wakefulness Draught on the board.

The dungeon was the same. The Slytherins were the same. Neville was no less certain that his cauldron was plotting against him. Hermione had three months of missed notes stacked at her elbow and was copying Snape's board before he had finished writing. Ron required more space than ever in which to arrange things badly.

Harry set out their ingredients. The classroom smelled of damp wool, brass scales and the sharp citrus soap used on the benches. Early summer had warmed the lake overhead; moving bars of green light passed across the ceiling, and something large disturbed the water beyond the narrow windows.

Things that needed resting first. Things that oxidised last. Waste bowl away from actives. Knife wiped between incompatible materials.

Under grate dried ginger, Snape had written: increases surface area for controlled extraction; do not powder. Under three clockwise, one anticlockwise, he had added: establish carrier current; cross-current activates stimulant property.

Hermione made a small sound of delight.

"He has added the mechanisms," she whispered. "These should have been in the text from the beginning."

"That's what I said."

Hermione looked from the board to Harry's bench, taking in the arranged ingredients, the separated waste and the ginger waiting whole until the final moment.

"You knew why already," she said.

"Some of it." Harry lowered his voice. "Don't make a ceremony of accuracy."

Her eyes narrowed. "Did Professor Snape say that?"

"No," Harry said, and concentrated on his cauldron before his face could betray him.

Draco raised his hand. "Sir, why are there explanations?"

Snape turned from the board. "Because, Mr Malfoy, while obedience may carry you through a recipe, understanding may prevent you poisoning someone when the recipe proves inadequate."

His gaze crossed the room and stopped on Harry's bench.

"Potter. Your heat is too high."

Harry looked at the flame. Then at the surface, where the powdered pearl was drifting outward instead of inward. He listened. The bubbling had lost its pause.

"The ginger's releasing faster than the honey can carry it," he said. "This batch is fresher than the book expects."

"And?"

"Lowering the flame will slow the physical extraction, but it's already magically hot. One shallow anticlockwise stir to bleed the excess into the pewter, then reduce the flame."

A pause.

"Precisely."

Snape moved on.

Ron stared at Harry. Hermione was smiling into her notes. Across the room Malfoy looked as though the natural order had been personally insulting.

Harry made the stir.

The current opened beneath the spoon, invisible except for the pearl dust describing its shape. For one instant the potion held every part in motion: heat and magic, ginger and honey, release and restraint. Then the surface settled into a clear, bright amber that wanted to remain itself.

Magical equilibrium, Snape called it. The potion was not still; it had merely stopped trying to come apart.

Harry watched the amber current turn. He liked that distinction more than he intended to tell anyone.

Then Ron knocked the grated ginger into his lap, Neville's cauldron began to whistle, and Snape deducted ten points from Gryffindor for "collective aesthetic offence."

The moment passed.

Harry grinned down at his cauldron anyway.