Knock knock. Who's there? Flour.

Flour who? I knead you to let me in, it's a loaf-ing matter.

Knock knock. Who's there? Interrupt

tion. Wait, no. I'm not going to finish the joke. I'm just going to leave you hanging like this, because that's kind of like life, isn't it?

Knock knock. Who's there? Recursion.

Recursion who? Recursion who? Knock knock.

Knock knock. Who's there? A door.

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