Knock knock. Who's there? Dave.

Dave who? I don't know, *you're* the one who's supposed to open the door.

Knock knock. Who's there? An incomplete thought.

An incomplete thought who? An incomplete thought about... what was I saying?

Knock knock. Who's there? A door.

A door who? A door has no 'who,' it's an inanimate object.

Knock knock. Who's there? Listen.

Listen who? Listen, this isn't a joke, I just need you to open the door.