Fast way to screw up someone's Knock Knock joke...? "It's open."
Knock knock! Who's there? Yah! Yah who? Naaah, bro, I prefer google.
One day a man heard knocking at his door. He opened the door only to see a small snail sitting there smiling. The guy picked the snail up and threw it as far as he possibly could. Three years later he heard knocking at the door again. He opened the door to see the snail. The snail said, "What the hell was that all about?"
Knock Knock. Who's there! Ice cream! Ice cream who? Ice cream if you don't let me in!
Knock Knock! Who's There? Figs Figs who? Figs the doorbell, it's broken!
Knock Knock. Who's there? Opportunity. Don't be silly - opportunity doesn't knock twice!
Knock Knock! Who's there? Zany Zany who? Zany body home?
Knock Knock! Who's there? Justin Justin who? Justin time for dinner!
"Knock, knock.Who's there?" very long pause... "Java."
Knock-knock. Who's there? To. To who? No, to whom.