Sherlock sucks on the inside of his cheek because, well, that was a joke, obviously yes, but there's... oh, if there isn't a bit of truth to it too. Not a hat, not anything, but...

"I want to take you apart," he blurts, soon enough after John has finished speaking that it might almost be an interruption. "Bit by bit and down to the smallest pieces and then put you back together again, so I can hold it all in my head, so... but I'm not going to, you see; couldn't possibly. Not even going to try."

Well... "Maybe after you're dead. Not good? No, not good. Not a hat. No hats. But they're not completely wrong, you see? You probably should be running. I want to know everything about you."

The rapidity of the words, how they come in bursts, a few here, a few there, interspersed with awkward pauses clearly indicates this as something Sherlock hasn't entirely intended to say, at least in such depth -- if, of course, the faintly mortified expression weren't enough. He feels heat rising to his cheeks and swallows down the sudden tightness in his throat.

"Should... know the worst about each other." A pause, and then quietly: "That's not the worst. Nearly the worst."