Compare with Jim's joke:

JOKER (suddenly): I just heard the greatest knock-knock joke. You start!

JERK (eagerly): Knock knock!

JOKER: Who's there?

JERK: Uhhhhh.....uhhhh....*I* don't know!!!!!!!!!!!!!!!!!!!!

JOKER: Sorry. This time, *you* say knock knock.

JERK: Knock knock!

JOKER: Who's there?

JERK: AAAAAAAAAAARRRRRRRRRRRRRRRRRRGGGGGGGGGGGGGGGGGGHHHHHHHHHHHH!

Needless to say, I was the "jerk" in this particular case. (It may have been in the '80s.) But this time, *you* say knock knock. You'll never guess the punchline!