Knock knock. Who's there? A door.

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

Knock knock. Who's there? Fig

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

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? Pane

Pane who? Pane-de-monium, I'm having a window of opportunity to tell you a glass-half-full joke!