Chekhov's Gun

Chekhov's Gun is the name given to a certain storytelling trope. Coined by the playwright Anton Chekhov, in can be stated as "if a gun is shown on the mantle in the first act, it must be used in the second."

The idea is that in good, economical storytelling, everything is important. If something with large potential significance is presented (like a gun), that should be foreshadowing the later significant use of that thing.

Sometimes the rule is deliberately violated to throw the audience off track and surprise them later, but generally it holds.

"Excuse me, Egon, you said crossing the streams was bad."