"Something's been bothering me." John's image floated in Sherlock's peripheral vision, sitting in his usual stuffed armchair and clutching a mug of steaming English Breakfast tea.

"Mmm?" Sherlock hummed, not looking up from John's laptop. The telly was on, but John's head was turned at an odd angle, so he was likely looking at Sherlock.

"You said... you could tell Irene was in love with you." This time Sherlock did look up. As he had predicted, John was fixing his flatmate with a curious stare. Irene was likely fresh in John's mind thanks to that little fib he'd told Sherlock last week about her being in some witness protection scheme in America. Absurd. Although he could understand why Mycroft wouldn't want John to know about Irene's new career in MI6.

John was still staring, waiting for a response. It was always so frustrating to deduce John's emotions. Sherlock could tell when John had suffered a nightmare (he'd been particularly restless after that case with the diamond smuggling ring), or when he was going on a date (the last time being seven and a half weeks ago, on a Friday, and he'd come home afterwards in a particularly foul mood), or what he'd eaten for breakfast (eggs, followed by toast with strawberry jam). But he could never tell why John looked at him this way.

"Did Mycroft tell you that?" Hopefully John was going somewhere interesting with this. The afternoon had been so dull.

John ignored Sherlock's prickly tone and barged forward.

"You can't tell love from... physical symptoms, Sherlock. I mean, physical attraction is one thing. Love is entirely different, isn't it? How did you know she wasn't just attracted to you?"

Sherlock rolled his eyes. Obvious. "She was a self-professed lesbian, John."

"Oh. Right. You overheard our conversation." John suddenly looked flustered as he considered the other implications of their chat. Irene had accused John of being attracted to Sherlock. 'Look at us both,' she had said.

"Yes, John. Besides, even if she weren't gay, there are actual physical symptoms of being in love that are distinct from the signs of physical attraction. It's almost impossible to maintain arousal for an extended period of time, without stimulation."

John looked oddly contemplative. "Is this just further proof that you need to sleep with someone?"

Sherlock sighed. Honestly, some days he didn't know why he bothered.

John doggedly continued. "No, it's just... People don't just fall in love overnight. How did you know for sure?" John was twitching, clearly irritated and trying to prove a point.

"Elevated heart rate. Dilated pupils."

"That's the same as for arousal!"

"Heart palpitations. Flushed skin colouring. Lack of apparent sexual desire."

John furrowed his eyebrows.

Sherlock smirked at John's frustrated expression. "Like I said, there are distinct physical symptoms."

"But love isn't… you can't…"

"Limerence."

"What?"

"The technical term is limerence, John." John was just looking at Sherlock, puzzled, his forehead creased, brows pushing together as though they wanted to form a single long strip of fuzz. "It's what most people refer to as being in love. Oh, for the love of – stop staring and look it up if you don't believe me!" He shoved the laptop towards John, who picked it up and carried it back to his armchair.

John, still radiating doubt, started typing out a web address. Sherlock knew when he had found the page even before the man huffed out a faint "Huh."

Sherlock simply waited, pretending to be sending a text message. It was always entertaining to try to deduce things about John without his knowledge. If Sherlock appeared to be doing something else, John rarely got suspicious, and the additional challenge of not using his sight in the analysis kept things interesting.

"Um."

The utterance made Sherlock look up with a jerk of his head. That had sounded… concerned. John wasn't looking at Sherlock, however. He was reading the Wikipedia page, and an expression of horror was blooming over his features.

"What? What is it?" Sherlock asked. This was most aggravating. He could deduce something was gravely wrong (any idiot could tell that, even Anderson) but he didn't know why.

John shook his head as if coming out of a trance, and his eyes refocused on his flatmate from across the room. He shut the laptop with a snap and shoved it onto the coffee table. Then he got up and started towards the kitchen.

"While you're up, could you make me a cup as well?"

"Yeah, fine," came the mumbled reply.

Sherlock leapt up and snatched the laptop from where it had been discarded. John's password, new as of Tuesday, but still patently obvious ("StayAwayFromMyStuffSherlock"), was quickly entered and the expected Wikipedia page for Limerence greeted him. Odd. Why would this provoke such a reaction in John?

Was he missing something? Some other evidence? Perhaps John had recognised symptoms in himself of which he'd previously been unaware. Unwanted limerence. Yes, that would explain several things about John's recently erratic behaviour. Clearly, Sherlock needed to determine the object of John's affections, and determine why it was causing him discomfort.

A purely selfless effort on his part, obviously. Sherlock had no more sinister motivations than trying to assure his flatmate's happiness. It didn't have anything to do with his own desires, certainly.

Sherlock decided not to dwell further on the subject. If John started displaying telling symptoms, he would dig further, but if not, well, it was no concern of his.

It took all of forty-seven minutes for Sherlock to give up on his policy of non-interference.

To his credit, he was without a case, and those forty-seven minutes had been particularly excruciating.

The biggest obstacle, Sherlock had determined, was to learn to recognise the symptoms of limerence without establishing direct contact. John would be suspicious if Sherlock attempted to obtain his pulse every time he talked to a pretty woman. And the base rate measurements would certainly be deemed "inappropriate."

Sherlock could picture it now.

Oh, I was just taking your pulse to make sure you weren't having heart palpitations. Yes, I realise you're a doctor.

No, I'm not trying to manhandle you. Why would you say that?

Of course I would never dream of conducting experiments on you that might affect your heart rate.

John, come back.

John?

Sherlock sighed. He would have to approach this carefully.

Unfortunately, heart rate was the most telling symptom. It would be difficult to establish with certainty that John was experiencing limerence without that data.

xxxxx

Sherlock started formulating a plan while John was in the shower. It was easier to think when the man wasn't staring at him or going on about something ridiculous like the weather or football or, God forbid, the solar system.

The first step was to establish a control group. Mrs. Hudson, Lestrade, and Sherlock himself were obviously currently unaffected by limerence, so they would be in the control group. Molly would be in the clearly limerent group, and John would be the test subject. Sherlock was fairly confident from John's reaction to the Wikipedia article that he would test positive for limerence, but it was difficult to tell without conclusive data. After all, it was only the case that John believed himself to be limerent, not that he actually was.

The second step would be to develop a hypothesis and then experimentally determine the truth of such.

Hypothesis: A limerent person experiences dilated pupils only when faced with the object of limerent desire, given a consistent testing environment.

Experiment: Observe Molly's pupils when in the presence of John, and in the presence of Sherlock himself.

It wasn't ideal (it would be difficult to factor out other potential causes of pupil dilation, even assuming fixed light levels), but it would be a start. Reasonably satisfied with his experiment design, Sherlock shrugged on a coat. "John! We need to visit Bart's."

John poked his head out of the bathroom, half his chin covered in shaving cream. "What? Why? Can it wait five minutes?"

Sherlock sighed, exasperated. "Time is of the essence, John!"

"I'm going to look like an idiot if I stop shaving now. I'll try to be quick." He returned to the bathroom's interior, and Sherlock could make out some muted cursing. He really ought to buy John a replacement razor. The experiment with the cantaloupes had dulled it considerably, and John tended to cut himself when he was shaving quickly, even with a sharp razor.

John emerged from the bathroom, bits of toilet paper peppering the right side of his chin, and a frown gracing his features. "I'm blaming you for this. There's no way I'm getting a date with this many cuts. And they're only on one side! They'll think I'm doing it on purpose."

Sherlock sighed. Really, why did John complain? It's not like these so-called dates ever went anywhere. "I'm sure it's the latest fashion, John," he replied dryly. "Shall we?"

"Yeah, let me get my coat on," John said, struggling into his jacket. Sherlock was tempted to help, if only so his flatmate would go faster.

xxxxx

When they arrived at Bart's, Molly was her cheerfully irritating self, and to separate himself from John, he cajoled him into fetching them both coffees.

Sherlock was pleased to note that Molly's pupils did, indeed, dilate in his presence. He tested this at several different angles to make sure, and then waited for John to return with the coffee before excusing himself.

He attempted, as stealthily as possible, to gauge Molly's pupils from a distance, but the window into the room was too far away to provide a clear view into the room, and Sherlock had to wait for several minutes while Molly's back was turned. Fine. He would have to resort to second hand data.

Are her pupils dilated? SH

After a moment of reflection, he came up with a marvellous idea, and sent a second text.

Are his pupils dilated? SH

John's response came first.

No, you daft git. Get back in here, she's moaning like a love-sick fawn. JW

Followed by Molly's.

No, I can't really tell. Maybe it's just the light in the room. It's a bit bright in here, isn't it? Are you expecting them to be dilated, or not? Maybe if I mo

Oh, I'm so sorry, I must have gone over the character limit. Do you want me to send that again? xx Molly

No. SH

Idiotic woman.

What did you send to Molly? She's been txting for the last 3 minutes. JW

Irrelevant. SH

John smirked at Sherlock when he came in, and Sherlock was interested to note John's pupils... dilating. Interesting.

Oh - he'd forgotten to observe Molly as he came in. "I forgot my riding crop, one moment." He strode back out, John chuckling at his back.

Well, that was a terrible excuse. His riding crop was safely at home.

"Never mind," he said, pushing the door back open, and looking at Molly's pupils intently, "I must have left it at Mycroft's."

He was disappointed to see that there was no change in Molly's pupil size. Perhaps she had been thinking of him while he was gone, and it had ruined the experiment? Or perhaps the lighting conditions were such that he was getting a false positive with John. Irritating. He'd have to try again with a more precise measurement technique.

"Come along, John, must retrieve it before my brother notices."

John merely raised an eyebrow good-humouredly, the edge of his lips twisting into a wry smile.

As soon as they were outside, John asked, "I seriously hope we never have to retrieve your riding crop from your brother's estate. Think of the questions."

"Questions?" Sherlock asked, puzzled.

John merely smirked. "You know... How long was it gone before you noticed?"

Sherlock frowned. "That doesn't seem like a particularly interesting question. It would depend entirely on my need for the riding crop."

"Exactly," John said, grinning. When he saw Sherlock's puzzled expression, the grin transformed into a scowl. "You're really thick sometimes, you know that?"

"Ah. One of my many attractive attributes, no doubt. I'll file that away along with 'spectacularly ignorant,' 'idiot,' and 'sociopathic tendencies'."

John tilted his head and smiled fondly at his flatmate. "Don't forget 'daft bugger'."

"Ah, yes, mustn't miss that one." Sherlock realised he was smiling benignly down at John and forced himself to direct his gaze elsewhere.

"Ah, John," he mentioned absently, "did you happen to notice my pupils? Control group, very important."

John smiled. "I don't know, they look dilated to me," he teased.

Sherlock looked back at his flatmate with a snap. "What?"

John poked Sherlock in the side. "Don't worry about it. Probably the light out here. It's awfully dim, don't you think?"

Unfortunately, John had a thoughtful expression on his face as he looked at Sherlock. That was never a good sign, in Sherlock's experience.

xxxxx

Sherlock had noticed John paying closer attention to him the past few days. He'd obviously caught on when Sherlock had asked about dilated pupils, and had been waiting with quiet amusement for the next phase of Sherlock's experiment. Clearly, asking John directly for data would be no more successful than reaching for his wrist to take a pulse; both would be regarded suspiciously and impact the test results.

No, Sherlock needed access to a monitoring system of which John would be unaware. He hit upon the solution unexpectedly, as often happened with his best work. Mycroft, the annoying git that he was, had come to the flat to check up on them, and ostensibly offer Sherlock a job.

Sherlock had, of course, told Mycroft exactly where he could stick his boring, idiotic case, and John had let out a dramatic sigh as Mycroft stomped off the premises.

"Good riddance," Sherlock sneered at his brother's retreating bulk.

John had just sighed again. He did have certain melodramatic tendencies. "Sherlock, you can't keep doing that to your brother. For one thing, we could use the money."

"It was obvious, John. Besides, I don't trust him. For all I know, it was just a ploy to plant more bugs in the flat."

John looked alarmed. "Wait, what? Bugs? You're not serious, are you?"

"Hmmm. He went to the toilet. I'll have to check there as well."

John turned a rather appealing shade of red and stammered, "Wait, you think he bugged the loo? What the hell is wrong with your family?"

Sherlock waved his hand dismissively, but he couldn't fight back the smile at seeing John's flush. "Don't worry, I'll be thorough. You won't have to worry about preserving your modesty from my brother's prying eyes. He'll be sorely disappointed, I'm sure."

John blinked stupidly for a few moments before turning about face and marching into the kitchen. He was probably hiding behind his kettle, as he tended to do when embarrassed.

As Sherlock was busy inspecting the toilet for hidden cameras, it occurred to him that he could plant his own cameras in the flat. Of course! It would be the perfect way to collect data on John. He could use the data to easily establish a baseline for John's skin tone, pupil dilation, and any other visual signs, and whenever someone visited the flat, he'd be able to easily tell how John was affected.

In fact, he could even use his current search for Mycroft's bugs as an excuse to install some of his own. Not in the toilet, of course (John's reaction implied that would be a Bit Not Good).

Brilliant! He clapped his hands in glee. It would be much better if he could tell John, of course, since the man never ceased to be amazed by Sherlock's good ideas, but that would defeat the purpose of the experiment. A shame, that.

xxxxx

Sherlock was not a patient man, and so the initial wait to analyse the data he'd collected from John was irritating, to say the least.

The first few days of footage needed to be aggregated and quantified in private, and there was only so much Sherlock could accomplish with software, advances in vision recognition algorithms notwithstanding. But John wouldn't leave. Surely he had a shift at the surgery coming up, but when Sherlock asked about it, John just raised an eyebrow and mumbled something about Sarah, forced time off, and annoying git of a flatmate.

Sherlock had been forced to come up with a few arbitrary experiments that used the remaining jam, so that John would be forced to go to Tesco's to purchase a replacement.

It took two days for John to discover the toes inside the jam jar (longer than Sherlock had expected, considering John's affinity for toast), but finally, on Sunday morning, John stalked out of the kitchen, grabbed his coat, and slammed the front door behind him, muttering angrily about no-good-flatmates as he went.

As soon as the door closed behind John, Sherlock leapt off the couch and rushed to his room to collect his laptop. Normally, he would just use John's, but there was a small chance John would discover the files. Besides, Sherlock had better video editing software.

With almost a week of footage from five separate cameras for Sherlock to sort through, he would have more than enough data for a preliminary analysis. He briefly considered using John's absence to place a camera in John's bedroom, but decided that the prospect of studying the existing data was more exciting than expanding his collection opportunities.

Sherlock shut his door and settled onto his bed. He impatiently fast forwarded through the collected footage. Most of the living room footage was of Sherlock, pacing about, playing the violin, or reclining on the couch in his favoured thinking position. He deleted it. Irrelevant to the experiment. John would come in for hours at a time, but he would usually sit typing on his blog, or with his focus on the newspaper, so his pupils weren't clearly visible. Useless.

The kitchen cameras showed both men at different times. Sherlock would go in to check on his experiments (he was only running three at the moment, since his experiment with John was far too absorbing to maintain his regular routine). John would make tea or toast or beans on toast, and occasionally scrambled eggs or the odd omelette, until Sherlock casually mentioned his fertilization experiment on Thursday morning, and John stopped eating eggs entirely.

The front hall camera had little useful footage, but did show John and Sherlock's comings and goings.

One scene in particular stood out. John and Sherlock had just come back from a visit to Scotland Yard (paperwork, dull), and John was saying something witty to Sherlock, who was trailing behind him. John's face was turned away from the camera, but Sherlock's was clearly visible, and the expression on his face was... Well, quite frankly, it was disturbing. He was practically beaming at John.

He deleted the footage immediately. After all, he was collecting data on John, not himself.

He saved all the footage that contained clear shots of John's pupils. Unfortunately, John spent so much time reading, looking down, or facing away from the cameras that in the end, there were only five hours of useful footage. No matter. It would be a good starting point.

Sherlock was startled out of his reverie by a sudden knock on his bedroom door.

"You all right in there? You've been cooped up for the past hour and a half, I'm starting to get worried..." John's voice was muffled by the door, but Sherlock could detect a trace of amusement. "I made tea," he added.

As if tea would lure Sherlock away from an experiment.

"I also bought more jammy dodgers; you seemed to like those last time."

Sherlock snapped his laptop shut and leapt off the bed. Curse John and his underhanded methods. Really, the man could be quite persuasive when he put his mind to it. Those fluffy jumpers concealed a calculating interior – one that knew the manipulative value of biscuits.

With some effort, Sherlock managed not to snort audibly at the thought, and he carefully masked the grin that had cropped up without his permission.

Once again composed, Sherlock flung the door open with a flourish, revelling in John's amused expression. "I suppose I'll come out. If only to stop your incessant jabbering."

John's smile grew wider as he turned back to the kitchen to fetch the tea and biscuits.

xxxxx

John left for the clinic the following Thursday, which gave Sherlock ample opportunity to analyse his collected footage. Unfortunately, all this gave him was a sample of John's normal state. Well, and the times he was with Sherlock, but that was practically all the time; he hadn't planted any bugs in the toilet, and he hadn't yet had an opportunity to plant any cameras in John's bedroom. At least, not yet. Sherlock made sure to remedy that once John had left.

Still, Sherlock needed a better source of data. One that would enable him to observe John when he was outside the flat, on dates, and such. Ever since Jeanette, John had seemed to give up on the whole dating thing. Or maybe he'd just stopped taking his dates home to meet Sherlock.

Sherlock frowned petulantly. If John didn't introduce him to his dates, how could Sherlock possibly determine if they were good enough?

That was, however, irrelevant to his current concerns. Sherlock needed to plant a camera outside of the flat, without John's knowledge. Although Molly would likely help him bug the lab, John was only ever there with him and Molly, and Molly had been ruled out as a candidate during the first experiment. No, wait – he couldn't use that data after all, as the conditions had been unsuitable. Too many variables. Still, who would fall in love with Molly? He shrugged and tried to think of other possibilities.

Going to the clinic where John worked would be suspect, and Lestrade would likely refuse to grant Sherlock access to the Yard's security footage. Again. And it's not as if he could bug the pub John went to with Lestrade for football matches. He'd already tried that; it didn't end well. Angelo would surely let him monitor the restaurant, but John only ever went there with Sherlock.

Which left only one glaringly obvious solution. Sherlock needed to use John's mobile camera.

John had a front-facing camera – of lower quality than the rear camera, but still adequate to capture video. It was a recent model with good internet access through the mobile broadband network. It would be a simple matter to install a program to turn on John's front camera remotely, and stream the data feed back to Sherlock's home server.

It wasn't often that Sherlock experienced gratitude, but right now he was quite thankful for Harry Watson's unlimited data plan.

When John arrived back at the flat, laden with bags from Tesco's, Sherlock stretched out a hand.

"John, I need to send a text."

John struggled in the door and heaved his bags on the kitchen counter, jostling a sensitive experiment on light absorption of decaying flesh.

"Those are delicate, you know," Sherlock huffed in irritation, but there wasn't much bite to it. He'd asked John to buy him more cantaloupes and another pack of jammy dodgers, and judging from the shape of two of the bags, John had delivered. Good, dependable John.

"It can wait two bleeding minutes while I put away the perishables." There were muffled thuds from the kitchen and then John poked his head around the corner.

Sherlock was still holding his hand outstretched, otherwise perfectly still and solemn, laid out flat on the couch, with his other hand resting lightly on his stomach. When John didn't move away from the doorway, Sherlock wiggled his fingers.

"Oh, all right then. What did you do this time, drop it in the Thames? Leave it in your other trousers? Or perhaps it's in your pocket and you just don't want to reach for it?" John teased.

Sherlock said nothing, merely waited for the cool smooth metal to hit his palm. As soon as it did, he snatched the phone and started typing furiously. He did have to send a message ("Piss off, Mycroft, and next time try to leave your cameras in less obvious locations"), but first, there was some software that he needed to install.

xxxxx

There was no point in having access to the camera on John's mobile if the man insisted on keeping his phone in his pocket.

Sherlock rather viciously fast forwarded through ten hours of darkness interspersed by two minutes of John's face, three and a half minutes of his ear as he shouted angrily at his sister, thirty seconds of his face as he read, but didn't respond to, another text message from Harry, and twenty-six minutes of the ceiling of the doctor's office as he worked on paperwork with his phone out of his pocket.

Ridiculous.

Sherlock would have to start texting John more frequently if he wanted better data.

He switched his laptop to the live feed (oh, brilliant, John's pocket again, what a surprise) and pulled out his Blackberry to shoot off a text.

Bored. SH

Sherlock saw the lighting change as John withdrew the mobile from his front left jeans pocket. A smile was hovering at the edges of his mouth, and on reading Sherlock's message, John's eyes crinkled up with mirth and his smile widened, showing the barest hint of teeth.

He set the phone down on his desk, so his face was now only partly visible.

Ha! Proof that John deliberately ignored him. He knew it.

A few seconds later, John picked the phone back up and started inefficiently pecking out a reply. Sherlock was painfully aware of John's texting speed, but the visual aid of John smiling seemed to be making the wait less aggravating than usual.

And what do you expect me to do about it, exactly? JW

Fascinating. John was still holding the phone, apparently waiting for Sherlock's reply. Sherlock wanted to see precisely how long John would wait before setting the phone back down and returning to his paperwork.

Thirty-seven seconds. Interesting.

Give me a case. SH

John peered at the phone, apparently having heard the buzz of the vibration on his desk. He smiled at Sherlock's message and stuck his tongue out the side of his mouth while he typed up a reply. Sherlock wondered if John realised he did that or if it were an unconscious habit.

Sorry, I think you have the wrong number. Unless you're looking to solve the case of the missing semi-skimmed, try Lestrade. JW

Sherlock suppressed a grin as he typed a reply. Seeing John's reactions was even better than he'd imagined.

Already solved that one. Mrs. Hudson was the culprit. She bribed one John H. Watson with the promise of scones. Very shady. SH

Sherlock stared at the laptop screen as he pressed send so that he wouldn't miss John's reaction. Something was coiling in his gut, warm and fluttering. His heart was beating faster. This feeling was almost as good as cocaine, or having a case; it gave Sherlock thrills to make John react this way, to see John when he thought he was safe, private, unmonitored. It was wrong, Sherlock knew, and if John ever found out he would be angry. So angry. An excited shiver ran through him at the thought.

John giggled and bit his lip when he read the message.

Are you planning on revealing these findings to the police? Or are you going to hold the evidence over the doctor's head in order to extort a share of said scones? JW

Do you take me for a monster? I will do neither. But if the good doctor feels my deductions merit a reward, I would not object to a scone or two. SH

Well then, if I see this alleged Dr. Watson, I'll let him know of your desire for scones. JW

John suddenly looked pensive, wistful almost, but Sherlock didn't get the chance to analyse the expression, as without warning the mobile was being put back into John's pocket.

No! It had been going so well. What had gone wrong?

Sherlock recalled the original purpose of his plan to text John; he had successfully collected more data. He should be happy that he'd been able to get as much as he had. But there was something gut-wrenching about John's face as he had slid the phone out of sight...

xxxxx

When Sherlock came out of his room a few hours later, he discovered John had returned from the clinic, as he was sitting in his armchair.

But there was something off. John was drinking tea.

Normally, this would not be of note, but for some reason Sherlock could not stop staring.

John's Adam's apple undulated as he swallowed, a few drops dribbling out of the side of his mouth, eyes closing in apparent bliss.

When he finally opened his eyes and noticed Sherlock staring at him, John paused, lowered the mug, and wiped off his mouth with the back of his right hand. "Finally decided to join me?"

Sherlock's gaze drifted to John's mouth, where there was still a drop of tea clinging to the corner. John really ought to wipe that off. "New tea?"

"It's got this sort of bitter aftertaste, but at least it was only one-twenty for a fifty gram box." John subconsciously opened his mouth and flexed his jaw muscles.

For some reason the motion made Sherlock deeply uncomfortable, and he looked away quickly. Something was pooling in his gut, a strange tension, and he had to do something to dispel it.

He strode past John's armchair, rushing to put on his coat.

"Is there a case?" John asked curiously.

Sherlock shook his head violently. "No. Going to the shops. I need oranges for the mould study," he added as an afterthought.

John brightened considerably at the word 'shops'. "Oh! Right then. Can you get some beans, while you're at it, and more jam, thanks?"

Sherlock scowled as he stomped out of the flat. Of all the idiotic excuses to leave, why did he have to pick one that was not only excruciatingly dull, but would require he bring back evidence?

xxxxx

When Sherlock got back to the flat, petulantly hauling three grocery bags with him, four of the six cameras he'd planted in the flat were sitting on the kitchen counter.

He seemed to have... underestimated John.

"Mycroft stopped by," John commented offhandedly, continuing to fill the kettle without turning and looking at Sherlock. "I thought I'd make sure the flat was clean. He said he wanted to invite us to some family dinner. Don't worry, I told him off."

"Oh. Thank you," Sherlock responded numbly. How had John known about the cameras? Unless he really did think the bugs belonged to Mycroft…

Sherlock's flare of hope was extinguished when John turned to Sherlock, steaming mug of tea in his hand. John's eyes were twinkling mischievously, and his lips were quirking upwards in a smile. He handed the tea to Sherlock, who accepted it wordlessly, stomach sinking like a stone.

"The funny thing," John mused, "was the number of cameras." He paused and took a slow sip from his own mug as Sherlock twitched uncomfortably.

"There was one in the front hall, one in the living room–"

Two in the living room, thought Sherlock sullenly.

"–and two in the kitchen. Now, I don't see why Mycroft would need more than one camera in a room. And it was quite curious. One of them was angled towards the cooker, and I'm not aware of you ever having used it." John took another sip of his Earl Grey and watched Sherlock with quiet amusement.

"Perhaps my brother is attempting to obtain footage of you, not me," Sherlock tried.

John snorted into his mug. "Right. What, does he fancy me?" He looked up at Sherlock sharply, eyes widening in horror. "Oh God, he doesn't fancy me, does he? I don't think I could survive the attention of more than one Holmes."

Sherlock frowned. "What do you mean, more than one?"

John rolled his eyes. "Just forget it." He pushed past Sherlock deliberately and strode out into the living room, fingers clutching his tea a bit tighter than normal.

How was Sherlock supposed to gather experimental data now? He pouted briefly before striding out of the kitchen to join John. At least there was still the last living room camera, and the one in John's bedroom.

xxxxx

When Sherlock presented John with the new watch, he eyed it with suspicion.

"What's this for?"

Sherlock cocked his head. "Well, I was the one who destroyed your watch last time. I admit that the experiment with the acid may have been slightly misconceived."

John rolled his eyes, but accepted the watch without further question.

Sherlock smiled. That had been almost too easy. But John was the type to accept Sherlock's bizarre behaviour without question, and that was one of the reasons Sherlock liked him.

So John took Sherlock's watch, and Sherlock was pleased to note that he had a new source of data. It was amazing what you could pick up from online tutorials these days. Do-it-yourself heart rate monitors were surprisingly easy to assemble, even with the addition of a wireless upload component. Sherlock had started wearing a matching watch himself.

At the end of a week's time, Sherlock cloistered himself in his bedroom while John was out at the pub with Lestrade. It was time to analyse his findings.

The best part of the analysis was the heart rate over time graph. He could observe John's resting heart rate, his standing heart rate, heart rate in the morning and how it varied over time. For some reason John's heart rate was slightly higher in the mornings and evenings, and Sherlock couldn't ascertain the cause.

He scanned John's bedroom footage for a correlation to his morning heart rate elevation. Although watching John masturbate was oddly fascinating, it probably crossed some sort of personal boundary. Sherlock fast-forwarded through the footage until John had dressed and put on his watch.

Most of the other changes were easily explained – sustained high during a chase, spikes when encountering one of Sherlock's experiments, or yelling at Sherlock. John had a lower heart rate at the office except during five to twenty minute periods, when he was likely being visited by an attractive patient. Or Sarah. No data when he slept, or was in the shower, as he would remove the watch.

Sherlock's graph was simpler. His heart rate was elevated when he was running, and lower when he was laying still and thinking. There was a jump whenever he was analysing data from his experiments or on a case, and a strange pattern in the mornings and evenings that would require further study.

His remaining living room camera, irritatingly enough, was focused on the sofa, so revealed more of Sherlock than of John, except for the times when Sherlock could lure John over to sit with him. Last night he had insisted that James Bond films were implausible and a waste of time, and John had forced him to watch Die Another Day in retaliation. Sherlock had quite enjoyed himself; he had stolen John's popcorn, complained about the film, and determined how close John would allow him to sit before moving away. He hadn't protested when Sherlock had pressed his thigh against John's, or when he had lain against John's shoulder, but when Sherlock slid his toes under John's leg, his flatmate had batted Sherlock's feet away and scooted over to the opposite end of the sofa. Sherlock had then tried to lay his head in John's lap, but that had just triggered a giggling fit and more shoving, before John abandoned the sofa entirely and ran off to the kitchen.

Sherlock had watched that footage three times before he realised that he wasn't paying proper attention to John's pupil dilation or skin colouring; he was watching John's smile, the muscles shifting underneath his shirt, and the way his hand brushed against Sherlock's.

When he noticed his reaction to the footage, Sherlock realised that he had missed something vital.

He scrolled back through the footage in a panic. This time, instead of watching John's face, he checked his own. Eyes, dilated. Skin, flushed. He checked the heart rate monitor data. Elevated pulse, erratic heartbeat.

He quickly checked the previous week's data. Elevated heart rate corresponding to being in close proximity to John. Dilated pupils whenever he was recorded speaking with John. Heart rate, skin tone, and pupils all returning to normal when John left for the clinic. Spikes in heart rate corresponding to text messages sent to John and analysis of video footage of John. Elevated heart rate while on a case with John; not merely while chasing the suspect through China Town, but also beforehand, while analysing the scene, with John present.

There was too much data to ignore the most probable conclusion.

Sherlock Holmes was in love with John Watson.