A/N: Johnlock drabble. It's been a long time since I last wrote fluff/sillies. Hopefully I haven't lost the knack (assuming I ever had one).

It was in the dim firelight in the lounge of 221B that the idea occurred to him. It was something that had been bothering Sherlock for some time, and it was as he sat opposite the doctor in their flat – John reading the morning newspaper with a cup of tea in one hand, Sherlock perched on the edge of his black leather armchair, long fingers steepled in front of him – that he decided to test his hypothesis.

'John,' he said suddenly, his voice cutting through the silence that, until then, had only been punctuated by the crackle of the fire and the occasional rustle of the newspaper as his flatmate turned a page.
'What?' John replied, not looking up until a second or so later, only to catch sight of the detective looming inches behind his newspaper, causing him to jump slightly in surprise. '—Bloody hell, Sherlock…'
Sherlock had fixed his intense and calculating gaze on John, who could only stare back with an expression of slightly wary confusion. Pale eyes narrowed, flicking back and forth between John's darker ones.
'… What are you doing?' John exhaled finally, lowering the newspaper onto his lap and setting down his teacup. 'If this is about you being bored again, you know there's nothing I can do about it, unless you want me to go out and commit serial murder – which I won't do, by the way—'
Sherlock ignored him, leaning closer still, his brow furrowing slightly as he curled the fingers of one hand around his wrist, silently counting the pulse. John pulled back a little, raising an eyebrow.
'…No, seriously, what—'
'Experiment. Shh.'
The detective snatched up one of John's wrists, pressing his fingers to the vein. There was a long pause as John met Sherlock's piercing stare with one of confusion and mild concern for the sanity of his friend.

'Your pulse is slightly fast,' Sherlock remarked finally. John exhaled an awkward laugh, glancing at his captive wrist.
'You surprised me, idiot, what did you expect? Looming over me like that…' he cast another glance up at the detective's face, only inches from his own. 'Speaking of which, do you mind…?'
'No.'
'No, of course you don't.' John sighed, turning away in mild exasperation.

A few moments passed.

'It's not slowing down.'
'What?'
'Your pulse.' Sherlock released John's wrist at last, curling his fingers around his own wrist once more, frowning at his findings.
'W-wha—…' John spluttered, lost for words as Sherlock's gaze locked on his again, looming closer still, his brow furrowed in thought. After a moment, he seemed to come to a decision.
'Don't… move,' he said finally.

John was pretty sure he couldn't if he'd tried. In a second, his entire field of vision was filled with those high cheekbones and dark curls of hair as Sherlock's lips met briefly with his. Through his surprise, John dimly noticed the slight clumsiness of the consulting detective's kiss, as though he knew the theory of the gesture, but hadn't yet had the opportunity to master it.
Sherlock pulled back a few inches, measuring his own pulse again with a frown. John stared at him, speechless.

'I don't understand!' Sherlock exclaimed finally, straightening up and glaring around the room in frustration. The doctor watched him weakly from his armchair.
'Neither do I.' He blinked a few times in bewilderment, then shook his head. 'Sherlock – what the hell was that about?'
Sherlock looked around at John as though just remembering he was in the room.
'…An experiment, John.' His lips pursed in that familiar, childish pout.
'Don't give me that,' John retorted, pushing himself up from his armchair. 'You… kissed me.'
'Your pulse was fast.'
John let out a laugh of disbelief. 'What?'
'Your pupils,' Sherlock continued, '– slightly dilated – could be the lighting, but I doubt it, going on the fact that they weren't while you were reading your paper, and the rate of your pulse after I got as close as I did.'
John stared back at the pale, probing eyes that were fixed on his, his brow furrowing.
'…My pupils? How long were you— Do you just make a habit of staring at me when I'm not looking?'
'—But, the reason I kissed you, John,' Sherlock interrupted, 'was because of this.' He held out his arm, palm facing upwards. Frowning slightly, John pressed two fingers to the vein in Sherlock's wrist. The pulse beneath his fingers was racing. He glanced back up at Sherlock, eyebrows raised. Sherlock met his gaze with an almost apologetic smile.

'This is what I don't understand. I'm not supposed to have a heart, you see.'
John shook his head in exasperation. 'Idiot.'

Sherlock opened his mouth to reply, but was cut short as John strode forward, seizing the front of his dressing gown to pull him down to a more reasonable level, retuning the kiss. The taller man froze for a second in surprise, but as John's lips moved against his, gently coaxing, he found himself melting hungrily into the kiss.

Neither of them heard the noise on the stairs as John's fingers crept upward to tangle in Sherlock's hair. Nor the sound of the door opening as Mrs. Hudson let herself in.

'Ooh-! I beg your pardon,' she said, catching sight of the pair who froze as she poked her head around the door. 'I— I'll come back later.'
And before John could offer a syllable in his defence, she was gone.

He let out a sigh.

'Oh boy, never going to live that one down,' he said, scratching the back of his neck awkwardly, looking away. 'People will definitely talk this time.'
Sherlock smiled in fond amusement at the way the doctor's ears turned pink.

'Might as well give them something to talk about, then.'