"Oh, come on, Sherlock. It could have happened to anybody!"
"Not me."
"Not even if they caught you off guard?"
"I'm never off guard." Sherlock said lazily from where he lay, stretched out on the couch.
"Oh, please. I get it- you're always alert, always aware. But even you could be caught off guard!" John protested, setting aside his laptop and shifting slightly in his chair to get a better look at his flatmate.
"No I couldn't. I couldn't and I won't. It just doesn't happen."
John Watson took that as a challenge.
For the next three days, he tried to catch Sherlock off his game in every variant imaginable. He tried faking spilling scalding tea all over him, tripping him, shouting out random phrases. None of them had any effect on the detective. He simply rolled his eyes and instructed John to 'chalk up another point for Holmes'.
It wasn't until day four, when they were standing at a crime scene, that John finally got his chance.
It was a simple case of vandalism, nobody hurt but an old widow's bank account. Sherlock quickly deduced the situation, but Lestrade instructed him to stay and answer questions and explain how he came to his conclusions so they could be used in court.
"-from the paint splatters at approximately four feet high, he had to have been at least-" Sherlock did not get to finish that sentence. Without any warning, John reached over and pressed a quick kiss to his lips before pulling away and resuming his previous position, acting as if nothing had happened. Sherlock stood frozen and silent, eyes wide, a small smile tugging at the corners of his mouth.
"You win." He murmured coyly to John, glancing at him with a smirk. Everyone standing around them (especially Anderson) watched with their jaws hanging low and a mixture of shock and confusion on their faces. "Anderson, close your mouth. You'll catch flies." Sherlock said coolly, resuming his analysis of the scene. John grinned smugly from beside the consulting detective, feeling triumphant. He could perform experiments of his own, sometimes, couldn't he?