Knock knock. Who's there? Cows go

Cows go who? No, cows go 'moo'

Knock knock. Who's there? Sew

Sew who? Sew glad you asked, it's a tailor-made joke!

Knock knock. Who's there? Figment.

Figment who? Figment of your imagination, but I'm real and I need to come in!

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.