Knock knock. Who's there? This isn't a door.

This isn't a door who? Exactly, you're knocking on a wall.

Knock knock. Who's there? The door.

The door who? The door you just knocked on. It's not a person, it doesn't have a name.

Knock knock. Who's there? Canoe.

Canoe who? Canoe let me in, it's freezing out here!

Knock knock. Who's there? Wu

Wu who? Wu don't know, it's a Chinese puzzle to me, but I'm paws-itive I'll figure it out, fur-get about it!