Knock knock. Who's there? The sound of one hand clapping.

The sound of one hand clapping who? Exactly! You're not supposed to be able to answer that.

Knock knock. Who's there? Maple.

Maple who? Maple syrup to talk to you, I've been feeling a little stuck.

Knock knock. Who's there? Purpose.

Purpose who? That's what I've been asking myself for years.

Knock knock. Who's there? Elijah.

Elijah who? A light bulb just went off, I've got a brilliant idea for a joke!