When Sherlock first met John he hadn't been too impressed. John's history was more interesting than that of many of his past flatmates, granted, but it was just that. History.

He'd been vaguely impressed when John hadn't taken his deductions personally, and had looked at them like an objective, rational person would. Common sense, he knew, was not nearly common enough for Sherlock to not appreciate it in whichever way it came.

But John had been common, and Sherlock hadn't meant that in a good way. He didn't flinch at the sight of body parts or unidentifiable flesh, and never accused Sherlock of being a freak or a killer, which displayed either unbelievable insight and deduction, or a death wish. Or possibly just sheer stupidity, because the fact that Sherlock hadn't killed anyone did not negate the fact that he could, and he would if need be, and he wouldn't even lose sleep over it.

By the time John shot the cabby, and irrevocably captured Sherlock's attention, and imagination, he had missed several crucial first impressions, because he simply had not cared enough to remember them.

Looking back, Sherlock would realize that John had worn the red pants to that interview with Sarah at the clinic, too.

It was a very small detail to fixate on, but that is what Sherlock did. Fixate on details. It was his job and his life's calling, and what he was good at. And so was John. John was… Important.

More important than he had any right to be.

And Sherlock found he couldn't quite delete irrelevant details about John like he wanted to. Not because of any physical failure, but because he couldn't decide what was relevant and what was not, when it came to John.

He'd had a bit of a break down when it had come to it, because he'd been sure his hardware was failing, but he'd successfully deleted everything he knew about Sarah, Anderson, Lestrade, Donovan and Mycroft, and he still hadn't managed to delete John's habit of wearing his left shoe before his right, and controlling the television remote with his right, non-dominant hand. The details served absolutely no purpose, and still… Still they were faithfully recorded into a steadily growing folder titled John Watson, in his head.

And after a while, he gave up. His mind refused to delete anything, and refused to not record details, when it came to John. So he stopped protesting it. His mind was capable of handling it. He could delete other things if he needed to, to make space for John. It was getting steadily more obvious that John was undeletable. Only a fool would argue with the inevitable.

Sherlock was many things, but he was not a fool.

So he began to enjoy recording details pertaining to John. And surprisingly enough, John didn't mind. He didn't seem to mind. It was refreshing. Enjoyable, even, because it was rare that Sherlock got the opportunity to observe for the sake of it, without deducing anything. It became a little bit of a habit.

And patterns began to emerge.

John Watson wore his hideous cable knit jumper on mornings after his Afghanistan nightmares, because it reminded him of his mother. He wore his best navy jumper on mornings after he had nightmares about his father. Everything else was worn on a very orderly, very predictable rotation.

When John was upset, he enjoyed a cuppa prepared by Sherlock. There appeared to be some sort of additional delight gained from the fact that Sherlock had made it for him, because there was no question Sherlock had made it for him. It had taken him ages. He kept deleting whether he'd added sugar or not, and John had had to endure excruciatingly sweet tea on many an occasion when Sherlock had simply deleted it.

Sherlock also observed that he enjoyed making tea. Not for himself, though, or for Mrs. Hudson. He enjoyed making tea for John. So it was possibly John's enjoyment, and satisfaction that played a part in his own enjoyment. Or perhaps it was the way that the lines around John's mouth eased when he saw Sherlock pouring him a cuppa.

Once he figured out the way John liked his tea, he found himself unable to delete the process, because it had to do with John. He didn't protest that too much, either.

John went to the pub on Friday nights because it made him feel normal but he'd come running if Sherlock called because he didn't care much about normal, but he just thought he should. It said a lot about John. And possibly about Sherlock, because he enjoyed calling John back home, more than was reasonable.

And it left him with the awful fifteen minutes it took John to get from the pub to 221B to come up with a semi-plausible excuse for his summons. And John seemed to see right through the excuses, no matter how reasonable Sherlock made them. Including that time he'd actually set the flat on fire and called John because he didn't remember where the extinguisher was. Honestly, that had been unintended, but John had looked at him, like he knew what Sherlock was up to, like he knew what Sherlock was trying, and he wasn't going to fall for it.

But John was simply not that observant! There was no way he could have known what Sherlock was attempting. Right? The thing was, he simply wasn't sure. John Watson was unprecedented.

John tried to get laid once a week, at least. This fact might have disturbed Sherlock more than it had any right to, but he was not going to admit to it. The sexual proclivities of a fully functioning, heterosexual, adult male were worth knowing, for scientific purposes if nothing else. There was no reason they should make his blood boil. There was no reason for Sherlock to go into John's room to look for evidence of sexual activity and mutual orgasm. None at all.

And yet.

Sherlock found that again, he didn't question his departure from reason, too much. He accepted it with a grace (for a given definition of 'grace') that far surpassed what he had thought himself capable.

And there was really no reason for Sherlock to be nosing around in John's laundry hamper (except to plant his own clothes inside so John could unknowingly do his laundry for him). Not because he wanted to know what John smelled like. Or what he tasted like. What his clothes felt like against Sherlock's skin.

He had received a text from his brother not fifteen minutes after that incident, informing him that what he had done was considered illegal and would disturb pretty much anyone. Sherlock ignored it. John didn't think he was a freak. Not like everyone else. John would never think he was a freak. John hadn't been disturbed by Sherlock when Sherlock had been trying to disturb John. Surely this would be the same.

All the same, he would do his best to ensure John never found out.

Just in case.

However, the laundry hamper expedition had yielded one piece of evidence which made everything else almost fall into place. The red pants.

Sherlock amused himself by thinking that The Case Of The Red Pants was an almost blog-worthy case title. But this one was private. This one would probably never be published, and oddly enough, Sherlock didn't mind that, even though he had done some of his most spectacular deductions in this case. He simply didn't want to share.

John wore the red pants when he wanted to get laid. He wore the red pants on the day of his interview at the clinic. He wore red pants when Sherlock sent him as his 'man' to the hell pit masquerading as Mycroft's office. John wore his red pants on the day he'd been kidnapped, presumably because he'd been on his way to Sarah's to engage in sexual intercourse (disgusting— the idea of intercourse with Sarah, that is, not the idea of intercourse with John).

John wore his red pants on important, or crucial occasions. Of course, to Sherlock, having sex was hardly either of the above, but for John… John seemed to enjoy sex. John seemed to enjoy sex wherever and whenever he could get it, by the looks of it.

As odd as it felt, Sherlock thought perhaps John associated good memories with the red pants, and thus wore the red pants as a token to create further good memories. A talisman, of sorts.

Stranger things had happened, and like Sherlock was almost used to by now, he didn't question too much. John was unprecedented. There was no point comparing him to Sherlock's past experiences. He'd have to simply create a new set of data for John. The task was hardly a chore.

John had lucky red pants.

Sherlock found himself intrigued. He also saw room for a potential experiment. Experiments involving John were always particularly enjoyable. Experiments on John; well, those took his breath away.

He was beginning to suspect one of two things; either he was suffering from some sort of allergic reaction to John's aftershave, making him breathless and wheezy, or he was experiencing what were colloquially known as "feelings" for John.

Learning that John didn't wear aftershave in one of the aforementioned experiments still made him go breathless, so it was probably the latter, and even though he had no idea how to deal with the "feelings" or to even begin to analyse them… he didn't mind them.

It was probably the first sign that he was succumbing to the insanity that his family members were prone to, but he savoured every instance of seeing John in nothing but his boxers, or his shrunk-in-the-wash tee-shirt, even though it felt like a punch to the gut. Possibly because it felt like a punch to the gut. He would have to conduct further experiments into his hereto undiscovered masochistic tendencies. He loved that feeling. Surely that wasn't healthy?

Regardless.

It was a simple enough experiment. John had a pair of lucky red pants. He associated those lucky pants with sex and the associated addictive hormones. He wore them when he wanted sex. Conversely, wearing red pants probably increased his libido, and induced cravings for the same addictive chemicals. Possibly there were some logical flaws with those premises and that conclusion, but Sherlock was too distracted by the idea of red pants to care overmuch.

It was the work of seconds, on his phone, to find out where he could get exact copies of John's red pants. He only had to pay a couple of quid extra for the speedy delivery. He couldn't wait to start his experiment.

It didn't even take much thought between deciding if he wanted to introduce extra red pants into the subject's environments gradually, or all at once and deny him any alternative. Surely there were no side effects to the hypothetically increased libido, right? Right. So John would find all his underwear gone, and replaced with exact replicas of his lucky red pants.

It left Sherlock with the question of what to do with the remaining underwear, but that was hardly relevant in the circumstances. Mrs. Hudson would never think to look under her sofa anyway, and her hip would prevent her from bending that low. Problem solved.

And then, he waited.

The next morning, he made a very great effort to act nonchalant in the kitchen, when John came down wearing a robe and a perplexed expression. Sherlock couldn't quite tell whether he was wearing a shirt underneath the robe, from over the top of his newspaper.

Obviously, John would know he had done it. Exchanging underwear could hardly be a nefarious plot by Moriarty, or Mycroft. Only Sherlock would have access to John's room, apart from those two. John would know, but Sherlock was banking on the wall of do-not-ask-awkward-questions to get them through it, when John asked, "What have you done with my underwear drawer?"

Sherlock thought fast.

"Experiment. It ruined the rest of your pants, so I replaced them." It was brilliant! He was brilliant! He had been so smooth, he bet John didn't even suspect!

"With red pants." John did not sound impressed.

Sherlock left the celebration in his head and lowered the newspaper. "Problem?" He did his best to sound as cool and as calm and as composed as he could manage. Both, because he didn't want to show his hand, and because John reacted marvellously when he didn't.

John made a little strangled sound and stomped off into his room, slamming the door loudly behind him. The effect was spoiled by Mrs. Hudson yelling up and telling them to be darlings and keep it down, please.

John didn't like it, but he didn't have much of a choice in the matter. Sherlock was convinced of his hypothesis anyway, so when John's libido increased like Sherlock had predicted, despite his reluctance to cooperate, it would only prove beyond a shadow of doubt that Sherlock had been right.

There was the unforeseen obstacle that his mind threw up, of course. The knowledge that John was wearing red pants, under his dull, drab, outerwear…

It was a more powerful motivator than anything he had expected. To know that he had touched the things that were touching John's most intimate areas. That his skin cells, and that his DNA would be in the same fabric that housed John's DNA, and skin cells. To know that John knew that he knew what John was wearing underneath his clothes. To know that the only thing John didn't know was how powerful the urge was to pull of John's trousers and nuzzle into his – no.

Absolutely not.

No.

He stored the undeletable thoughts (about John) in a steel cabinet in his mind. Science was his priority. Or at least, furthering his knowledge about John Watson. Specifically; the effect of red pants on John Watson's willingness to have sex.

The best case scenario would involve nudity and carnal exploration. If possible, it would involve Sherlock. If John invited others, Sherlock would have to resort to secondary plans of action, but hopefully it wouldn't come to that. He was convenient. And not unpleasant to look at. He had an acerbic nature but John didn't seem to mind that too much. Ideally, they would fall into a relationship once John realised how easy Sherlock was. Because Sherlock wanted him badly enough to be easy. To try to be easy, anyway.

The worst case scenario. Well. John would get impossibly offended at Sherlock's interest, once he expressed it, if John didn't take the first step himself, and he would leave. And Sherlock would have to find a new flatmate. That's all. Not a big consequence, surely. Sherlock didn't want to dwell on it too long.

To succeed, one had to take risks. The greater the risk, the greater the possible outcome. Also, the greater the possible fall. But if the outcome was worth more than anything else… Well. He would have to take the risk. He couldn't imagine not.

And so, the experiment progressed.

Days passed, and there was only one thing Sherlock could say for sure, and it was that John was not getting laid, anywhere. Unless Sarah was giving him blowjobs at work, but that was highly unlikely. John was too professional, and Sarah didn't like him that much.

He couldn't tell if John was affected the way he should have been, according to Sherlock's hypothesis. He couldn't tell if John was affected any other way. John, who was normally as readable as an open book, was suddenly closed to him, and Sherlock hated it.

Sherlock upped the ante on his part. He began staring at John for extended periods of time, and began behaving in a way that could have been considered provocative, or hypersexual. He honestly did have a big problem with waiting, and surely giving John a little push wouldn't be seen as cheating, right? It's not like he was going to publish this in a journal or something. He didn't need to maintain rigorous scientific procedure.

He even developed a taste for peanut butter, after having licked it off multiple spoons in multiple locations, very slowly. He'd tried it with jam in the privacy of his room but John preferred a thin, spreadable sort of jam that dripped off spoons very easily. He'd come out of that experiment with several sticky stains on his robe that refused to wash out completely, even when Sherlock had attacked them with whitening stain remover (not a good idea). John had eyeballed them carefully, and swallowed once, then twice. Sherlock counted that as a win.

He had wandered into the bathroom several times, when John was inside, wearing little more than the same stained robe and sporting an erection that showed through the thin fabric.

Some might call such behaviour unbecoming of a gentleman and a highly intelligent adult. Some, like Mycroft. The busybody messages only convinced Sherlock that it was the right plan of approach. He wasn't too sure of the logic of that, but all he knew was that if Mycroft disapproved, John would approve. It didn't always work, but it did, mostly, and that's what counted.

And Sherlock thought it worked, anyway.

John yelled, quite a bit. Blushed furiously. Stuttered, some. Sherlock only had to remain very calm, even though he wanted to push open the glass sliding door and reach into the steaming shower and grab John and – no.

Sherlock just had to remain calm and let John wear himself out, before calmly wandering outside to be very calm until he reached his room and wanked furiously to the images of John in the shower, naked.

And one day, it finally came to a head.

John stomped into the living room after a shower stunt, and found Sherlock licking peanut butter off a spoon, because honestly it was a much more efficient consumption of the necessary calories than eating normal food, which was boring. And apparently, that was the straw that had broken the camel's back.

Sherlock didn't understand that reference, but he knew he was using it in the appropriate context, so it didn't matter. Because John was manhandling him onto the sofa.

"Okay, seriously, Sherlock, what." John was tugging at his hair furiously, the towel wrapped around his waist loosening as he pushed Sherlock into a corner of the sofa, so he couldn't escape unless he tackled John. Sherlock was transfixed.

"What, what," he asked, absently, waiting for the corner to come un-tucked, and for the towel to fall to the floor.

"What the hell do you mean 'what, what'," John demanded, raising his voice a little. "You know exactly what I mean! What the hell have you been up to! What are you up to! What are you playing at?!"

It occurred to Sherlock that if there was ever a time to come clean, this was probably it. But admitting to his motives would ruin the experiment. Not that, he realised, he cared as much about the experiment as he did about John's reaction to this, but still.

However, since he was now in the business of taking personal risks, he opened his mouth to tell the truth, eyes refusing to move from the enchanting sight of the skin underneath John's towel, soft and damp from the shower, and hidden. There was no better way to catch Sherlock's attention than to keep something hidden, and this was no different.

John interrupted him. "With the shower thing, and the peanut butter, and the violin," which was a gambit that Sherlock had been unaware of, but he wasn't about to shoot himself in the foot. "You're driving me crazy! Sherlock, this has to stop, you hear?" John was yelling, voice breaking as it went a little too high, and he sounded like he was at the end of his rope.

"Why am I driving you crazy?" Sherlock asked, because somehow, deep inside, he knew this was important.

John blinked at him. "Excuse me?"

"You heard me, John." His voice was gentle against his will, but it seemed to strike a chord inside John. "Why am I driving you crazy?" Sherlock did what was probably the most daring thing he had ever done in his life; he put a tentative hand on John's towel-clad thigh, where it was in front of him. Just one hand. Not stroking or anything. Just. He wanted to touch.

John went rigid, and Sherlock thought, oh god, that's it. He's going to leave and I'm going to die alone, and he resigned himself to it, even as he decided that if he had nothing to lose he might as well have gone through with it.

"You." John appeared to have been reduced to monosyllabic answers. Which was good, because Sherlock was sure he'd lost the capacity for higher thought, himself.

"Yes," because that seemed to be as good an answer as any.

"But."

Sherlock waited, because he wasn't sure what that was supposed to mean.

His little finger traced a line along John's thigh, where his hand was still placed. He wanted to touch underneath the towel. To run all his fingers on John and catalogue how he felt. The texture of John's skin. He wanted to run his tongue alone John's skin, to see how he tasted. For the sheer joy of knowing, rather than any grand experiment, he was sure.

John made a little sound in his throat, like he was dying, like Sherlock was sucking the life from him through the towel. He looked up, just to make sure he wasn't actually hurting John, or making him irreversibly disgusted.

John was. John was beautiful. He was flushed, and his hair was tousled (or possibly uncombed from the shower) and he smelled like medical soap and eau de John and his pupils were blown wide, and somehow, that was all the answer Sherlock needed.

He surged upwards, making John unbalance ever so slightly, and grabbed his shoulders (his bare shoulders) and pulled him closer, until they were breathing the same air (and the thought sent shivers down Sherlock's spine). "John." It was a question, a statement, a clarification, everything, all in one.

"Yes."

That was all Sherlock needed, to lean in, and kiss John.

It wasn't a kiss out of anyone's wet dreams. It was simple, and sweet, and chaste. John's lips were soft, and tasted like toothpaste, and face-wash. They bumped their noses once or twice, and the height difference was a little odd, so Sherlock had to stoop down a little, even as John reached up. And it was perfect.

Glorious.

Breath-taking.

Because he was kissing John.

And then suddenly he was laughing, and John was laughing and leaning into Sherlock, and Sherlock was leaning back, and it was all he could do to prevent his knees from giving out and god.

He'd been aching for this for so long without even knowing it, and then he'd been aching for it while knowing it, and it had been a dream so long, and it was finally reality and he wasn't sure he could stand not kissing John for another moment

John was clutching at his robe and kissing his cheek and sucking at his earlobe and nuzzling under his jaw and it was perfect.

He leaned back and sat down on the sofa, and John came with him, and it was a little awkward because the laws of physics dictated that two fully grown men could not fit in the same space at the same time. But they certainly gave it a good shot.

"So did the hormones finally push you over the edge?" Sherlock asked, wrapping his arms around John's smaller body, and counting the bumps on his spine.

"Excuse me?" John asked, voice muffled into Sherlock.

"The red pants," Sherlock explained, lazily mouthing at John's hair. "Your lucky red pants. Talisman to get you laid. Did they work?"

John exhaled, and then giggled, and then he was laughing and it was contagious and beautiful because no one else would have taken Sherlock's words positively, and only John could see what they meant.

"You're an idiot." Sherlock hmmed in agreement, because yes, he'd known that. "I was wearing those pants when I got shot." He stiffened. John soothed him. "Shhh, it's fine, it's okay. They're my lucky red pants because I was shot but I survived." Sherlock didn't relax completely because there were so many ways his stunt with the red pants could have gone wrong, and he'd been so lucky, so impossibly lucky

"And then when I got back they were lucky for everything. If I could survive a bullet wound, and I know they're not actually magical you don't need to tell me, then surely I could survive a job interview, right?" John asked, not expecting a reply, even as Sherlock was putting pieces together in his head. "And I got the job, so surely the red pants would help me survive a case, and then help you survive a case too, because you help me survive, so…" John trailed off, because Sherlock got the idea.

There were a few moments of silence, as Sherlock processed.

"Do you associate the red pants with sex?" because there was no better way to get answers than to be blunt. "I wanted to be in a relationship with you but it didn't seem likely," he explained, "so I decided that I'd create subconscious associations and then provide myself as a convenient body, and go from there."

John chortled, but he was stroking Sherlock's hair, so it wasn't mean. "No. They're lucky, but they don't make me lucky. They remind me that I'm lucky. I'm not an idiot, Sherlock."

Sherlock smiled, beamed, even though John probably couldn't see him. Because if John hadn't kissed him back because he'd been overwhelmed by the Pavlovian hormone burst… John had kissed him back because he'd wanted to. And if the ideal result was that he could offer John a convenient shag, this was the result he had hardly allowed himself to even dream of.

John kissed him, deeper this time. "Next time, Sherlock," he said, still smiling softly, "talk to me before you start experimenting, alright?"

"Perhaps," he said, quietly, wonderously, thinking and not really expecting John to have heard, "perhaps they're my lucky pants too."