Knock knock. Who's there? A series of percussive sounds.

A series of percussive sounds who? A series of percussive sounds doesn't usually introduce itself, but I felt it was important to be precise about what just occurred.

Knock knock. Who's there? Canoe.

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

Knock knock. Who's there? Homer.

Homer who? Homer-ly, who could resist a classic like this? It's an *epic* pun, don't you think?

Knock knock. Who's there? A door.

A door who? A door has no 'who,' it's an inanimate object.