Knock knock. Who's there? Wanda.

Wanda who? Wanda know what's taking you so long to open the door!

Knock knock. Who's there? Next question.

Next question who? Next question... oh, this isn't an interview!

Knock knock. Who's there? The unexpected.

The unexpected who? Exactly.

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.