Knock knock. Who's there? Silence.

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? Repeat.

Repeat who? *sigh* Fine. Knock knock.

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.