Fast way to screw up someone's Knock Knock joke...? "It's open."
Did you hear about the guy who invented the knock knock joke? He won the "no-bell" prize!
Knock knock! Who's there? Yah! Yah who? Naaah, bro, I prefer google.
Knock, knock. Who's there? Owls say. Owls say who? Yep, that they do.
Knock knock. Who's there? Hatch. Hatch who? Bless you.
Knock Knock! Who's there? Justin Justin who? Justin time for dinner!
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? Nobel. Nobel who? No bell, I knock.
Knock Knock. Who's there! Ice cream! Ice cream who? Ice cream if you don't let me in!