Knock knock. Who's there? Philosophically speaking, the notion of a 'who' is a social construct and the concept of identity is fluid, so the idea of a fixed entity on the other side of the door is a fallacy.

Knock knock. Who's there? Fig

Fig who? Fig-ure it out yourself, I'm busy!

Knock knock. Who's there? An anticlimax.

An anticlimax who? Exactly.

Knock knock. Who's there? The door.

The door who? The door you just knocked on. It's not a person, it doesn't have a name.