So here they were, the two of them, sitting on the couch, Sherlock on the armrest with his hand on John's head and John on the middle seat with a bemused expression on his face. Sherlock was very much intrigued and excited and fascinated for absolutely no reason at all, there was no logic to this tingling sensation he was feeling - feeling, as in through his five senses as well as through his mind. John's head was pleasantly warm and round and smooth and firm and his hair was not too fine but not too thick and Sherlock liked how the short hair strands pushed against themselves as he stroked and ruffled them. The faint sound they made as they shifted under his hand was wonderful, and reminded Sherlock of a field full of tall reeds blowing in the wind. The texture was fine, too. He could do this all day and not be bored. Definitely not bored. In fact, all the sensory information he was getting was too much. For him senses were used not to feel, but to know. Now he was both feeling and knowing, and the feelings he was experiencing was overwhelming him in such a way that he had no words to express them.

He could just about manage this:

"I must experiment and find out the length of time the strength of friction force need be applied to produce significant wear and tear on human hair."

"And where would you apply the amazing knowledge you gain, exactly?"

"Good point. The results will be useless."

"Obviously."

John's quip was full of silent laughter and Sherlock nearly smiled, then they were silent once again.

Sherlock felt John's occiput with his thumb, his palm, the back of his hand, with one, two, three, and four fingers, then with two hands and with the underside of his wrist. Same different; that's how it felt. Anyway it was all John and therefore all fine. He explored the hair whorl and traced the pattern of gold from it and felt what the border areas between hair and uncovered skin was like. Data, so much data, none of them really useful or applicable but how can he ignore these? He couldn't stop himself. He kept on.

After quite a long while, John spoke up, his voice lower than its usual tone.

"I think I should've set a time limit before agreeing to do this."

Sherlock hummed a noncommittal noise and wondered once again at the discrepancy between John's words and the tone and didn't miss the tired note in John's voice; yet he couldn't process anything as just gathering the sensory information was straining his system.

His hand paused for a fraction of a second, held up by an idea blooming, flooding, spreading his being: Maybe he could go beyond the occiput. Follow it down and end up at the area he had devoted all his attention to recently. And gain some much-needed tactile data.

Finally, he thought.

So Sherlock's hand moved down, following the curve, dipping down and onto the straight and smooth line that forms the back of John's neck, finally making contact after gathering all the visual information available with the most thoroughness only he himself can muster - and at the exact moment of full contact, Sherlock found himself on the floor staring at the ceiling in numb shock.

Oh.

He blinked.

OH. Of course. Afghanistan, army doctor, trained for combat, survival instinct, automatic reaction and all that, how in the world did he miss - no, forget - this he can't say for the life of him, of course John will react like this if someone suddenly touched the back of his neck without warning, and really how did he not deduce this he can only excuse himself by saying he was not focused. His occiput ached a bit from the forceful meeting it had with the hard, bare floor and John was fussing over him thinking maybe he was badly hurt since he was not responding at all, but Sherlock just stayed there, feeling calm, lying down facing the ceiling and thinking how relaxing all this was, and wondering why this was so relaxing, and also considering just what was it about John that made him let his guard down, and then going back to muse upon the strange riddle of John Watson's neck and how it thwarted his effort in every turn he made, and finally to ask himself why he wasn't frustrated at all with the lack of progress he was making but was feeling content instead. Questions, questions, but deprived of question marks and curiosity and the burning, all-consuming desire to know, full of something else entirely. It was all so soothing and peaceful and he was lying down and felt strangely unlike his usual self but in an inexplicably positive way. And Sherlock fell asleep. Just like that. He sunk right into the world of deep dreamless sleep.

John had a hard time half-carrying, half-dragging him to his room but he was persistent and careful and concerned and of course Sherlock could deduce that this was so when he was conscious again in the morning. Then he began thinking, bathed in the faint morning light, staring at nothing and focusing on something beyond everything.