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The first time she did it, it was on a whim.
Amy had an early-morning department meeting and was up and ready to go to work while Sheldon was still in the shower. Before she left, she opened the bathroom door to call out a goodbye, but then saw the steamed-up mirror above the sink and opted to leave her parting message there instead.
'Have a good day! Love you!' she scrawled with her finger before quickly retreating.
She wasn't sure why she did it exactly. There was just something about the idea of Sheldon stepping out of the shower, all naked and dripping, to see her little surprise note that tickled her. She drove all the way to Caltech with a smile on her face and then forgot about it for the rest of the day until she got home. The bathroom mirror was pristine by then and Sheldon never mentioned anything about it, but as she brushed her teeth in front of it that night she found herself smiling again, hoping he had liked it.
The second time she did it was just a few days later, that weekend. She got up early Sunday to meet her mother and aunt for breakfast and once again her departure time coincided with Sheldon's scheduled bathroom time.
'Have fun today! See you later!' she wrote this time, and drew a heart at the end for good measure.
When she returned that afternoon, the mirror was clean once again and Sheldon was out with the guys. She didn't think anything more of it.
The third time was on the following weekend. Amy was cleaning up from breakfast when Sheldon went to take his shower. While he was in there, Amy slipped in to write “Looking good!” in the mirror steam before slipping back out, grinning. She then settled on the bed with her iPad and waited for her turn to use the bathroom. A few minutes later the water turned off and Sheldon stepped out wearing just a towel, with a bottle of glass cleaner and microfiber cloth in hand.
“Here, since you insist on dirtying up our mirror, you can clean it this time,” he said, foisting the cleaning items onto her.
“You didn't like my messages?” she asked, blinking up at him.
“It's bad enough the bathroom here isn't well-ventilated, the mirror is sure to corrode from all the condensation build-up. I would prefer not to think about all the germs and oils from your hands smeared on its surface too.”
Amy sighed. Of course that's what he would think about, practicality over sentimentality. Of course she probably should have known that.
“You're upset?” Sheldon asked, surprising her.
“No,” she said, but then paused. Sheldon wasn't good with reading emotions, but here he was making an effort. It didn't help either of them if she wasn't honest. “Well, maybe a little.”
“Why?”
“I know it's silly,” she explained. “But I guess I wanted us to be the type of couple who leaves little messages like that for each other. I thought it was romantic.”
“Romantic? Smudgy mirrors are romantic now?” He threw up his hands. “Good lord, I'll never be able to keep up with all this nonsense!”
Despite herself, Amy couldn't stop from laughing a bit. Her poor, beleaguered boyfriend. He really did try to keep up with her romantic needs, she knew this. She could meet him halfway here, maybe find some other way to leave him love notes, maybe in his lunch.
“Okay, you're right,” she said, standing up. “I'll clean up.”
“Thank you!” he called after her as she strode into their bathroom and got to work.
When Amy stepped out of the shower a few mornings later to see an equation written out in the mirror's fog, she was surprised. And annoyed. She quickly wrapped herself up in a towel and stomped out to find Sheldon.
“Oh, so I can't leave you notes on the mirror, but you can use it as an extra whiteboard for your work?” she asked.
“Didn't you read it?” he asked, looking up from where he was working on his laptop.
She opened her mouth, about to fire back that of course she read it, but actually she hadn't. Not really. She hadn't even put her glasses back on yet. She had only seen enough to know it was an equation, not words.
“Come on.” Sheldon took her hand and led her back to the bathroom, passing her glasses over once they were both inside. She put them on and gave the message another look.
(∂ + m) ψ = 0
“The Dirac equation?”
“Oh good, you know it.” Sheldon looked at her expectantly.
“Sure, the quantum entanglement of subatomic particles,” she continued. Sheldon kept staring at her, his eyebrows raised, his head inclined toward her. She stared back.
“Good lord, you're going to make me actually say it!” he said after a few seconds passed. “Two microscopic particles can interact in a way that their quantum states become intertwined, so much so that the particles stay linked together even when a great distance comes between them.”
“The initial interaction causes the two separate particles to become one system,” she summed up for him, smiling. She saw it now. How two separate things can interact and affect each other in a profound and lasting way. How one could even cause the other to set aside the important principles of order and cleanliness in order to leave a love note on the bathroom mirror. “That's awfully sentimental of you, Dr. Cooper.”
“Yes, well,” Sheldon sighed and bent over to retrieve the bottle of glass cleaner from the cabinet next to the sink. “I blame the entanglement.”