Knock, Knock! Who's there? D umbbell. Dumbbell who? Dumbbell doesn't work so I had to knock!
Knock Knock. Who's there! Ice cream! Ice cream who? Ice cream if you don't let me in!
Chuck Norris was once part of a knock knock joke. The Joke ended abruptly when after the first knock the door blew up killing the man behind it.
Q: Why can't you tell blondes knock-knock jokes? A: Because they leave to go answer the door.
Knock Knock. Who's There? Justin. Justin who? Your justin time to wipe my ass!
Knock Knock. Who's there? Opportunity. Don't be silly - opportunity doesn't knock twice!
"Knock, knock.Who's there?" very long pause... "Java."
Knock Knock! Who's there? Justin Justin who? Justin time for dinner!
"Have you heard my knock-knock joke?" asked the blonde. "No," said the brunette. "Okay," said the blonde, "you start."
Knock-knock. Who's there? To. To who? No, to whom.
Fast way to screw up someone's Knock Knock joke...? "It's open."