Sherlock does try to keep a straight face. He does. He tries to be earnest, to play at confusion, as though he doesn't see what's so funny about any of it, obviously they're made for it -- and really, growing old and irritable in a shack together, that bit, that doesn't sound so bad... though why it doesn't sound so bad is something he'd rather not get into at the moment and so the words don't come.

Instead, he breaks into that smile, the real smile and not the Real People smile, the crooked one. Can't help it, really can't, and it's better than trying to explain that he meant it, parts of it. That that, at least, has to be true, has to happen, because he doesn't know what he'll do with himself otherwise. Even he knows that's nonsense. Utter nonsense. Only it doesn't feel nonsensical.

Regardless, it's good, this. Good enough for him to lean his shoulder against the wall and watch John laugh, just laugh, which is brilliant, isn't it? Sherlock doesn't make people laugh, not as a rule. He thinks he's hilarious sometimes, true, it's just that most people don't seem to agree. This is new. This is excellent.

"You alright? Never going to see that shack if you carry on like this, you know. You do need to breathe sometimes. Just think what you'll be missing if you don't."