Sherlock is balanced on the fourth floor banister of an open stairwell, Lestrade is shouting like he's someone's mother, Anderson is shouting like anyone cares what he has to say, Donovan's death-grip on the railing is all that's restraining her from pushing Sherlock over, and John is laughing.
It's all inconsequential noise, except for that last. It isn't precisely John's carefree laugh, but it isn't his 'I'm laughing to keep from shouting' laugh either. He's in a wide stance with hands hovering to either side of Sherlock's legs, laughing up at him like he's never seen a more ridiculous stunt and he's halfway tempted to try it himself.
Knowing John, that's quite possibly the case.
Sherlock has never had a friend; not the real kind. He has had many acquaintances and often been laughed at, but seldom kindly. He's never seen the point in self-pity. After all, he can silence his critics easily enough, and he's capable of enough cruelty to send the most vicious heckler running off in tears. He was never helpless against 'friends' like Seb. He chose them, used them for his own purposes every bit as much as they did him—for company, for distractions, for an extra set of hands, for a warm body. For his own entertainment. He got what he wanted, they got what they wanted, and it was, in the parlance of people who live that kind of lifestyle, a fair trade.
This, though, this open unself-conscious joy in the face of Sherlock's existence, is something new under the sun for him. No one but John has ever given him this: not his 'friends,' or his enemies, or his habitually subtle relatives, of whose affection he may be assured, but that doesn't mean they know how to have a good time.
He grins breathlessly down at John. "You'd better hold on."
"What? What-?"
John yelps and grabs his shins, and Lestrade and Donovan both start swearing a blue streak, as Sherlock bounces up on the balls of his feet and manages to hook his fingers onto the ledge at the base of the fifth floor gallery. "There, you see? Someone of approximately six feet could have managed it, or a brave man of 5'10"." One hypothesis proven right; he may as well see how many he can rack up. He tilts his chin down to John. "Would you like to try?"
"No!" Lestrade shouts, overriding the expression on John's face that's hovering halfway between dubious and amused. "You've proved your point, now get down before I have you arrested for tampering with a crime scene!"
John doesn't wait for arguments. He grabs Sherlock's waist, yanking and stepping back at the same time, bringing Sherlock's weight down on him and taking it like a short blond supporting column. It feels disorienting and weightless and risky, and now Sherlock is laughing too.