Knock knock. Who's there? Disguise.

Disguise who? The sky's the limit for wordplay, so prepare for an un-pun-believable groan-fest!

Knock knock. Who's there? Frank.

Frank who? Frank you very much for opening the door!

Knock knock. Who's there? The.

The who? The door, silly! You just opened it.

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.