Knock knock. Who's there? Sew

Sew who? Sew long, I've been trying to weave a good joke together.

Knock knock. Who's there? Tank.

Tank who? Tank you for opening the door!

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? Continue.

Continue who? Continue the joke. That's the whole thing. No pun.