Knock knock. Who's there? Elijah.

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

Knock knock. Who's there? Existential Crisis.

Existential Crisis who? I'm not sure, but the meaninglessness of this joke is a reflection of the void that lies at the heart of our existence.

Knock knock. Who's there? A philosophical question.

A philosophical question who? If a joke falls flat in the forest, and no one laughs, was it ever funny?

Knock knock. Who's there? I forgot

who? Oh wait, I remember now, but it's not like it matters because I'm just going to forget again anyway.