"How often?"
A question that is completely irrelevant, not in context, out of nowhere. John can be like that and he is aware of this, yet each time this happens Sherlock is thrown off his course. He has to pause, think, and trace the trail John was following.
Consider: The setting. 221B. The living room. Both sitting in respective sofas. Both with laptops. Himself browsing possible reactants he can use for his current experiment, John reading something online. The reading content is not necessary at this point though can be deduced later on.
Few pointers, but not quite enough.
Consider the past five minutes. What did John do? He concentrates and reminds himself the signs he picked up unconsciously, something he does with John. A habit he formed in the past few months.
Ah.
"Whenever necessary."
"Guess that means you don't remember the 'how often' part."
He doesn't answer. There is no need to answer since John knows how to read his silence, and he can hear the soft chuckle which shows that John does know indeed. Putting his laptop aside, he gets up from his seat and walks up to Sherlock. Sherlock can feel his approach, and while staring at the screen, seemingly engrossed with his reading, is alert in every possible way. All the cells in his body are straining, excited to find out what will John do now. For he has ceased to guess John's actions, stopped altogether, as he realised the joy of finding out. With John, this is better than deduction. He likes to make discoveries in real-time rather than in Sherlock-time. So he waits for it, tense and fearful and expectant and-
John ruffles his hair, ever so briefly, maybe for about 3 seconds.
"You and that mop of yours. Doesn't it get in the way? It's amazing you don't set it on fire or anything with all the chemical stuff you do."
Sherlock hopes that John will not be able to read his silence, just this once. Then John peers at him, looking into his eyes, smiling in that slightly exasperated way of his, and with his hand pushes back the black curls covering Sherlock's forehead.
"Can't see your eyes properly sometimes."
A moment of blankness, of total unconsciousness, where his mind just blows up, obliterating itself and just leaving sensations. Of warmth. Simple, pure warmth. Flowing into his eyes. Leaving an imprint on his skin.
Soon the hand is gone, the eyes are gone, and John is in the kitchen. Everything is back to how it was. The unpredictable has passed. He regains his mental footing but for the life of him cannot remember this one thing: Did he remember to breathe?