The first time he kissed her, it was for purely investigative reasons. He was a curious man with many interests, who hated to be kept wondering about things without a way of coming to a definite answer. He had often suspected that Watson would be a good kisser, but alas - he had never known for sure.
The occasion followed a lesson in deductive reasoning concerning the physical telltales of lying that he had insisted Watson endure, late on a Thursday night. He had visibly surprised her by offering up more than just one true information about himself and his past that had been formerly unknown to her. She in turn had surprised him by accurately identifying 19 out of his 20 lies.
The only one she was unable to spot had been about his dislike for Gouda cheese. He had to admit that perhaps his hatred for the dairy produce was not strong enough to register very accurately on his face.
Either way, the little exercise had put himself in the unique position of staring straight into her face for over an hour. He had watched as every correct guess had brought a glint to her dark mahogany eyes, every fleeting indignation had made her muscles twitch and every surprised inhale had flared her nostrils. Although they had lived under the same roof for some time now and had been in constant close contact, he still managed to find out new things about her every once in a while. Now he couldn't help but pay close attention to the remarkable features of her skull, the soft line of her eyebrows, the sharp jut of her cheekbones and the striking arc of her lips.
As a highly logical and educated man, he did not find it hard to admit that she was beautiful. Despite the slight cross of her eyes and the asymmetry of her jawbone, she could quite easily be described as one of the most attractive women he had ever crossed paths with. And so reason followed that he needed to eventually apply his deductive skills to his own bodily reaction to her stunning outward appearance. He was quite certain that he would be aroused by her, if he allowed himself the sentiment. He did not think of it as shallow - in fact, he thought human sexuality to be one of the most intriguing studies of all. Including and especially regarding, of course, his own.
It was not much of a move at all. They were seated face to face in the kitchen of their shared brownstone, their knees touching, his hands placed loosely on his own thighs while hers gripped the sides of her wooden chair, leaning in as far as she could in order to get a read on him. His height advantage allowed him the necessary radius so that all he needed to do was close the gap.
He believed that you did not need to be a highly intelligent and thouroughly educated investigator to deduce that the kiss surprised her. He kept his eyes open to observe all the parts of her face and physique he possibly could. If he had thought about this before, it would have occurred to him to capture the scene on video or hang a mirror in the adequate spot, so that he could examine this case study from all angles. As it was a rather spontaneous decision, he had no alternate mechanisms to assist him.
Her eyebrows lifted, her eyelids fluttering with rapid blinks, her breathing momentarily stopping in the midst of an inhale when his mouth met hers. Her left hand lifted off the chair, as if reaching for him - whether to stop him or to pull him closer, he had yet to deduce. However, before she performed either action, the hand dropped down again. Maybe it was because she had gotten used to his surreal excercises and sudden mood swings and wanted to indulge him, maybe because she had a great deal of self control, or perhaps even because she did, as a matter of fact, enjoy the kiss. He took note of all three possibilities and moved on to his next clue.
He knew that she could - and more importantly, would - stop him immediately if she wanted to terminate the event. He would never forcefully or manipulatively coerce someone into such an inherently intimate and potentially sexual behavior as a kiss, even if merely for observational practices. If she made the slightest indication whatsoever that she wished to abort the maneuver, his personal and professional ethics - as well as what little he believed to have of what other people might call a 'good conscience' - dictated that he must retreat immediately.
She did not, however, employ such a move.
Instead, he concluded confidently that the act physically - and quite possibly mentally - pleased her. Watson's eyes closed and the pressure against Sherlock's lips increased minimally. Her eyebrows dropped back down, almost to their relaxed state. Her breathing, however, stayed uneven and erratic, her heart clearly beating faster than her usual pulse rate. Sherlock was quite satisfied with the deductions so far. He had expected very similar consequences beforehand.
So far, all that had been touching were their lips, parts of their noses and the cotton-clad fronts of their knees. Sherlock was an expert in conducting experiments. He knew that the number of variables needed to be reduced in order to gain accurate results. It also offered him the opportunity to add to and thereby change the orginal setup and monitor their responses.
He started by letting his right hand rest lightly on her shoulder. Her reaction was immediate and very enlightening. A soft moan escaped her lips, which then parted ever so slightly, turning the mere pressing-of-mouths into a far more active kissing business. He deviated from the first probing contact further by sliding his hand slowly up her neck. He resided there for long enough to take her pulse, noticing his own picking up speed as well, before scraping his fingers tenderly up into the soft hair at her nape.
His assistant-slash-companion rewarded him with a decidedly hot flick of her tongue against his that he found himself describing in his head as 'lasting not nearly long enough'.
It was then that Sherlock realized that he had largely underestimated his own response to the whole ordeal. Taking into account her attractiveness, her likeable personality, her well-maintained constitution (concerning her physical fitness and self-care, mostly in the form of deodorant, perfume, body lotion, hydrating water intake, mouthwash etc.) and their friendly companionship, he had been fairly certain that he would know exactly what to expect.
He had been off by several units of measurement.
He found that a mere short probe of her was not nearly satisfactory. His next kiss was much deeper and almost visceral, involving the bite of his teeth and the sweep of his tongue. Her palm pressed against his chest, not to push him away, but to balance herself as she leaned in even closer. She was very responsive to his ministrations and in turn his palms were getting warm with heightened blood circulation and added perspiration. His breath tended to stutter every time her tongue reached out or her lips changed the angle.
Frissions of pleasure raced down his spine and directly into his groin, hormones like dopamine and adrenaline - the human body's very own drugs - bombarded his system. Colorful flashes danced behind his eyelids, which he hadn't even realized were closed. He opened them again in hope that it would calm the storm inside of him, but her sight had quite the opposite effect. By now, a pink flush covered her cheeks and parts of her neck. Her freckles hypnotised him, the scent of her white tea perfume rose through his flared nostrils straight into his brain. His heart rate and temperature increase were well above what he would normally experience with one of his other female 'companions'.
The only thing he could compare it to was, well, being high. And in the echo of his own rushing mind, he had to admit that he had missed this feeling - the mind-numbing and sensitizing sizzle of a shoot-up. He had long ago given up on ever feeling like it again.