Light had always enjoyed learning new things. He didn't even realise at first that his responses were unusual; that most people didn't feel a warmth in their stomach when they came across a new fact, that the other boys of his age were masturbating to thoughts or pictures of semi-naked people, instead of to articles in the encyclopedia. It made him a star pupil; he'd happily sit and read the textbooks, or listen intently as the teacher explained, instead of letting his attention drift as the rest of the class did. But it only happened with new facts; and as the teachers started going over things he'd already learned, Light became more frustrated.
He'd read every factual book he could get his hands on; theories were no good, the facts had to be proven to turn him on. He had a moment of hope that he might be normal when he tried masturbation, but it was just learning what the physical sensations of a hand around his cock or fingers up his ass were that turned him on, not the sensations themselves. For a brief time, he experimented with other people, finding out what they liked, but once he knew how they'd respond they weren't new any more, and he couldn't get it up. It was embarrassing, and he could never think of a good way to explain - "It's not you, it's just that I was never reacting to you in the first place" didn't seem like something he should admit. He gave up hoping that he'd find someone who turned him on as much as learning did, and started looking for more new information.
The Death Note gave it to him, in spades. He knew that Ryuk thought it was the deaths that he was getting off on, but really, it was the new facts. Being able to test theories and discover things for himself - things that no one else knew - it was the hottest thing he'd ever done. He'd lie in bed thinking of new ideas, and new ways that he could test them, and he'd never been so satisfied in his life.
Misa was a problem in some ways, because she really thought that he could respond to her, but she brought him new information - shinigami could be killed, how shinigami could be killed - and he would pretend for her in return for that.
The first few days he was cuffed and held prisoner by Ryuuzaki (at his own request), he spent curled up to hide his body's reaction - this is what it feels like to be cuffed, this is what sleep deprivation does, this is how humiliating it is to be turned on in this situation and know that Ryuuzaki is observant enough to notice it.
When he gave up the Death Note, he was even more embarrassed, since he could only think of one explanation for why he'd ask to be imprisoned - and he really didn't want to think of himself as such a stereotypical teenage boy that he'd do anything to get off. He tried to ignore the frustration he felt, back to not learning anything new and only hearing theories and probabilities.
He wasn't entirely successful. His attention kept turning to Ryuuzaki, each little fact he discovered giving him a rush, just because it was so hard to learn anything about the man.
This is how he looks when he doesn't want people to know that he's annoyed.
This is the way he reacts when he doesn't want to believe something, but does anyway.
This is how he studies someone when he's interested in them.
Finally, one morning, he woke up to find himself cuffed to the bed, with Ryuuzaki sat just out of reach, reading something on his omnipresent laptop.
"You have spent a great deal of time watching me, and are overly pleased with yourself whenever you discover something," Ryuuzaki began, not bothering to look up and check if he was awake. "I believe I have an idea of why."
"Let me guess, it's because I'm Kira," Light said sarcastically, waiting to hear how this affected his percentage.
"Actually, no," Ryuuzaki said, eyebrow rising slightly as he finally deigned to meet Light's eyes. "My idea will be quite simple to test."
"Really?" Light didn't trust the look on Ryuuzaki's face. "How?"
Ryuuzaki's lips twitched ever so slightly. "Have you heard of gravimetric analysis?" Reading the answer in Light's expression, he smiled. The smile was unnerving.
"What does this have to do with Ki- the case?" Light asked suspiciously.
"Absolutely nothing," Ryuuzaki freely admitted. "I simply wish to test my idea, and feel that it would be best to avoid using anything connected to the Kira case for fear of contaminating the results." He looked back at the screen, and cleared his throat. "Gravimetric analysis, which by definition is based upon..."
Light tried to ignore him. He tried not to listen to the steady stream of facts, new facts, that he hadn't known. He tried not to let his body react the way it always did.
Ryuuzaki glanced up at him after a few sentences, raising his eyebrows at the erection that hadn't been there before he started reciting a college lecture, and continued. Light tried to breathe quietly so that he didn't miss a word of what Ryuuzaki was saying. This was so much better than being in a lecture hall - he didn't have to worry about sitting still to keep people from wondering what was wrong with him, he didn't have to find a pose that would hide his response.
A response which Ryuuzaki seemed to have been expecting, although his only reaction to it was to speak a little more clearly, which made Light bite back a groan and try not to thrust his hips too obviously.
"The following calculations - are you sure you wish me to continue?" Ryuuzaki interrupted himself, blinking ingenuously at the furious glare Light threw at him. As if he needed to ask, when Light was desperate and panting just from listening for this long. "- calculations would be done for..." Light closed his eyes, letting the words stir him back to his previous state of excitement. He didn't even notice that Ryuuzaki had started stealing regular glances at him, taking in the sweat glistening on his chest, his straining erection dripping precum, his restless shifting as he tried to reach and touch himself even though it was clear that the only stimulation he needed was the facts Ryuuzaki was reading out to him. Very clear, from Light's rising breath and jerking hips -
Light froze as he came, slumping with relief and turning to watch Ryuuzaki with lazy, satisfied eyes as he finished the lecture with barely a pause.
There was silence in the room for a few minutes, before Ryuuzaki smiled again. "No wonder Light-kun shows no interest in girls - although this does raise questions about his relationship with Misa." Holding out a hand to stop the angry retort Light was about to give, he leaned over. "The lecture I just recited is part of a series. I have the earlier ones memorised." Light's breath caught, his cock twitching as if he hadn't just come. "Would you like me to fuck you while I recite them?"
Sex while learning new information. Light had fantasised about that, but even though he'd found a couple of girls (and one young teacher) who were willing to go along with his kink, they never found anything to teach him that he didn't already know.
He held Ryuuzaki's eyes, and spread his legs in silent invitation.