Knock knock. Who's there? Marmalade.

Marmalade who? Ma'am, I'd like to spread some cheer with a joke that's jam-packed with puns!

Knock knock. Who's there? Existential Crisis.

Existential Crisis who? I'm not really sure, because the concept of identity and self is fleeting and meaningless in an infinite universe.

Knock knock. Who's there? This isn't a door.

This isn't a door who? Exactly, you're knocking on a wall.

Knock knock. Who's there? Who.

Who who? No, YOU'RE asking 'who.' I'm 'Who.'