He doesn't often indulge himself these days; but if he's going to spend the rest of the day with his family, then he definitely requires a cigarette.
His parents are deep in conversation with Molly – discussing Arthur's endeavours, most likely – and that provides him with the perfect opportunity to sneak outside. As he lights the cigarette he idly wonders whether Mycroft is going to join him, like he did on a similar occasion; they talked about dragons that day, and he ended up slaying one only a few hours later.
Christmas is a bizarre time of the year for the two of them; he remembers the cigarette his brother offered him at the morgue, right after the Woman faked her death. The two of them are never going to get along, and yet they'll always be there for each other, no matter what.
He throws a bored glance at the sitting room window, his eyes growing suddenly intent as he's presented with the most unexpected of scenes.
Mycroft is crouched on the floor, pretending to be interested in his nephew's favourite book. A planet might as well leave its orbit, Sherlock thinks wryly; he's never done anything of the kind for his little brother, but it seems that middle age has been softening him somehow.
He imagines Arthur teaching his uncle the names of all planets, and he all but laughs.