The corner of Sherlock's mouth tugs up in a smile, the crooked one, the real one, and he snorts. Almost laughs along with John, except John isn't laughing. Either he's really getting better at this lying business, or...

Sherlock's expression quickly reshapes itself into one of confusion. That doesn't make any sense. The only person ever to say anything like that to him is his mother, and they're meant to, mothers. Fathers are meant to too, but his never did, which is why he knows it must only be duty which motivates his mother.

So it doesn't fit, it doesn't make any sense, even if John isn't like anybody else and therefore couldn't, logically, be expected to react like everybody else all the time.

"You're serious. Are you serious? Why are you serious?" It doesn't fit, not really. But he has a feeling that his reaction is likely to be as laughable as the rest, so he attempts to cover it with dryness. "That was supposed to be the bit where you... I don't know. Run screaming? That's how it normally goes. Give you another chance, never know when I'm going to decide to skin you and wear you as a hat, or whatever it is people usually assume."