To flog a dead horse, I've found another joke told in a movie. THE PURSUIT OF HAPPYNESS ends with this exchange:

Christopher: Knock knock.
Christopher Gardner: Who's there?
Christopher: Nobody.
Christopher Gardner: Nobody who?
[Christopher doesn't respond]
Christopher Gardner: Nobody who?
[Christopher still doesn't respond]
Christopher Gardner: A-ha-ha, that's a good one, I like that!

(Just seen it. Great movie.)