A few years ago, we watched Despicable Me and Despicable Me 2 with my brother. After the second movie, Blaine observed: “In the first movie, they said, ‘I don’t know why anyone would ever want a fart gun.’ But in this one, it turned out to be really useful.”

We all laughed, but he was right. In Despicable Me, Dr. Nefario misunderstands Gru’s request for a dart gun and invents a fart gun instead. He tests it on a minion and then tosses it aside saying, “I was wondering… Under what circumstances would we use this?” But in the sequel, the fart gun proves key to defeating El Macho.

That’s an example of a Chekhov’s gun, a memorable but seemingly unimportant detail introduced early in a story whose significance becomes clear later. It gets its name from the Russian writer Anton Chekhov, who advised storytellers: “Remove everything that has no relevance to the story. If you say in the first chapter that there is a rifle hanging on the wall, in the second or third chapter it absolutely must go off. If it’s not going to be fired, it shouldn’t be hanging there.”

Larrabee is re-reading one of the Harry Potter books now, and they are full of such details–from the Dumbledore card that Harry finds in his very first Chocolate Frog on the Hogwarts Express to Regulus Black’s locket. Particularly on a second reading, he spots these relevant tidbits with a delighted “aha”!