A kiss.
It was so fast and so sudden, and John was so preoccupied with the newspaper, that he was caught off-guard. Sherlock just lunged at him, so that one moment he was staring at the face of some poor murdered woman, and the next - that of his flatmate, very close up indeed. John dropped the paper.
"Sherlock- what the- I thought you- what?"
"Experiment. Thinking. Shut up."
"Sherlock!" Maybe he should've left right then, slammed out of the flat like so many times when Sherlock pissed him off but.. He was stunned into his seat.
Sherlock moved in once again, still wearing the almost-emotionless, calculating expression he got when he was deducing something, which wasn't exactly a rare occurrence.
This time, John jerked backwards, away from the encroaching mouth, upsetting his mug off the arm of the chair. Neither of them registered the sound of smashing china. Neither of them moved for several hours- no, seconds. Just, very long seconds.
Finally Sherlock made another quick pounce, and this time, despite the soldier's best attempts, Watson was unable to escape.
He quickly became aware that he did not want to.