Knock knock. Who's there? Syntax error.

Syntax error who? Please try again with a valid input.

Knock knock. Who's there? Maple.

Maple who? Maple syrup to talk to someone, I'm feeling a little drained.

Knock knock. Who's there? (Knock knock.)

(Knock knock) who? Oh, for goodness sake, you just started the joke again! This is an endless loop!

Knock knock. Who's there? Someone.

Someone who? I don't know, I just heard a knock.