Lestrade had a favorite saying around Sherlock: "If you're going to act like a child, then I'm going to treat you like one." Over the years this had changed from a threat to an actual lifestyle…
Everyone and their mother knew that Sherlock didn't eat while on a case but Lestrade learned quickly that slipping his friend anything sugary—chocolates, cookies, literal packets of sugar—would result in their disappearance.
Naps were good—Sherlock's brain almost never shut down for an entire night.
He threw temper tantrums, became manically ecstatic over the simplest things, his favorite question was, "Why, why, why?"
Basically Sherlock was a child in a man's body and Lestrade had resigned himself to playing father.
On today's agenda, John was recovering from a stab wound and Sherlock refused to leave his side. However, an unconscious John was boring, so Lestrade had popped a movie in before Sherlock could start terrorizing the nurses. Some film called Up. All Lestrade knew what that his kid nephew was obsessed with it and if it was good enough for him, it was good enough for Sherlock.
In actuality of course, Lestrade had expected disaster. Instead he came back to find the credits rolling and Sherlock bouncing excitedly in his seat.
"Lestrade, quick! An experiment. I need an abandoned house and 10,000 balloons!"