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? Dictionary.

Dictionary who? I'm not defining the terms of this conversation.

Knock knock. Who's there? Egg

Egg who? Egg-cuse me, I'm cracking under the pressure to make a good pun.

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.