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It was a spell that did it.
Dean and Castiel had been more or less fooling around with inventing new spells because Castiel was taking a class about spell creation for his PhD, and even though Dean wasn’t taking the class, he found it really fucking interesting.
Not that it was that far outside the realm in which Dean typically worked. Dean was working towards his own graduate degree in Magical Engineering, with the aim of one day creating racing brooms that far outstripped the competition. He often came to Cas for advice on how he could tweak the properties of spells and magical objects.
“I’m just saying, I feel like I could make the broom a lot faster if the handle were thinner.”
“Yes, but Dean, then you would need to expand the cushioning charm in order to allow the rider to be comfortably seated, which might possibly negate the effects. The broom itself might be faster, but once you add the cushioning charm and the rider…”
“No, you’re right. In a vacuum, the broom will go faster, but with the external forces working on the broomstick in order to actually make it ride-able--”
Castiel and Dean had met at Ilvermorny School of Witchcraft and Wizardry, where they discovered at the age of eleven that they each carried the other’s soul-mate mark. During their seven years of magical education at Ilvermorny, they were inseparable. First as best friends and later as boyfriends, soul-mates all the while.
When they finished at Ilvermorny, it was only logical that they continue on, since Dean wanted to specialize in Magical Engineering, and Castiel dreamt of being an expert on magical theory. They enrolled together at Brown University’s magical division, which was of course, a secret to the innumerous no-majs that were to be found around campus.
This is all really quite secondary to the primary story, which is of course how Dean and Castiel ended up with a telepathic link.
Castiel, being born into a no-maj family, had the idea first.
“Dean, it’s really inconvenient that witches and wizards don’t use cell phones. What if we wanted to be in immediate contact, but we weren’t near a fire? Or we had to rely on owls? A Patronus is too noticeable, and really if you wanted to have a private conversation, impractical.”
“Cell phones would be great, but you know they fizzle out after a couple of days too close to magic.”
“Yes, but I had an idea.”
And so Castiel and Dean attempted a spell that would allow them to speak to one another in their minds when activated. Like--
“An invisible cell phone? Cas, you’re a genius!”
“Not if I can’t get it to work.”
“You will.”
And so Castiel wrote the spell.
“We have to say it at the same time, Dean. And make sure that you’re thinking about the idea, you know, that we can hear each other but only when we want. We don’t want to hear every thought that travels through each other’s minds, think of the noise!”
“I remember. Audi nos, right?”
“Right. On the count of three? One… two… three!”
There was a flash of bright purple light, and both men found themselves flat on their backs in the middle of their dormitory. Cas put his hand up to his head.
“Merlin, that fucking hurt!”
Cas sat straight up. That thought had not come from him. That could only mean--
“Dean?”
“Cas?!?”
They stared at each other, utterly speechless.
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They soon discovered that their link, as they called it, could be activated by sort of “thinking” a conversation in the same way that they might be able to speak a conversation; though occasionally, some unfiltered thoughts came through or they forgot to respond aloud to a question and caused some confusion, especially when among other people, who usually expected their conversational partner to respond out loud.
“Dean, did you hear me?”
“Of course I heard you, Charlie. I answered you, didn’t I?” Dean was annoyed, before he realized what had happened.
“Uh. No?”
“Oh. Sorry, I thought I did. I meant to say that yes, I enrolled in the same Physics class as Cas. You did, too, right?”
“Yes! This is going to be awesome. Literally the only class we all have in common, and the schedule gods have allowed us all the same time slot. Praise be to the schedule gods! Let’s go learn about physics and then go learn about how we can break the laws of physics by bewitching magical objects!” Charlie was nearly shouting, but her enthusiasm was catching.
The other effect that Dean and Castiel noticed was that evidently, their link meant that they stared at each other without speaking aloud quite a lot, which was an endless source of amusement to their friends, who couldn’t possibly know that inside their heads, their two friends were having a conversation about what type of groceries they needed to buy after school.
“I was thinking about making burgers tonight.”
“You want burgers every night.”
“Because they're delicious, Cas.”
“We need to eat more vegetables. I'm picking up salad ingredients.”
“As long as there is meat. And cheese. And as long as you don't pick up any fruit to put in there. Fruit does not belong with a salad. Who thought putting strawberries in a salad was a good choice?”
“Dean, you liked the dried cranberries last time I put them in a salad.”
“An anomaly, Castiel!”
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“Cas, should we tell someone about our link?”
“You mean that it exists, or that we can’t seem to turn it off?”
“Well, both. Or either. I don’t know.”
See, that turned out to be a problem as well. Castiel’s spell had had the effect they wanted, but they couldn’t turn it off. They could block each other out for a time, but the link remained. Neither of them could be particularly bothered that they could talk to each other privately. It was a small thing, and it wasn’t as though they weren’t soul-mates. Their link wasn’t going to get in the way of possible romantic relationships, because it only served to enhance theirs. They gradually got used to the link, and their lives returned to business as usual.
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Dean did not realize that their physics professor would be old as time itself, and he did not realize that when your teacher was a dinosaur, it made paying attention in class very very difficult.
Fortunately, he had a way to keep himself entertained.
“Do you think that Professor Stone is called that because he was literally around when the first stone was invented?”
Cas rolled his eyes. “I hardly think stones were invented. I think they just existed.”
“Yeah, but what came first? The stone or the professor?”
“I don’t care. If I thought it would liven up this discussion, I would throw actual stones at this human stone.”
They simultaneously snorted with suppressed laughter, and Charlie gave them a suspicious look.
“Are the laws of thermodynamics actually funny, or did I miss something?” She whispered over to Dean.
“Don’t worry about it. Cas is just being an idiot.”
“I’m being an idiot? You actually just asked me if our professor invented rocks.” Cas glared at Dean.
Dean didn’t dignify that with an answer.
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Castiel forced Dean to swear that he would never use the telepathy to cheat off of Castiel during a physics exam. Dean, for his part, was slightly offended.
“What makes you so sure that it’s me who needs to do the cheating? Why can’t it be you that doesn’t know the answers to the test, Novak?”
“Because I study for the exams and know the material backwards and forwards, Winchester.”
“Oh it’s on now.” Dean opened his textbook. Castiel grinned.
This lead to Dean and Castiel both being more prepared for the upcoming physics exam than they had been for any previous exams. They both wanted to prove the other wrong about who would need help the most during the exam.
Sat in alphabetical order by last name for the purposes of evaluation, Dean and Cas were not seated anywhere near each other. So, when they both bust out laughing, their professor was not amused, and assumed they were texting under their desks.
He was baffled when he found that neither of them even owned a cell phone.