Knock knock. Who's there? Donut.

Donut who? Donut even think about glazing over this punchline; it's sure to be sweet!

Knock knock. Who's there? Byte.

Byte who? I'm having a little trouble processing your question.

Knock knock. Who's there? Tank.

Tank who? You're welcome! No, seriously, *tank* you for opening the door; I was starting to lose my *bearing* out here.

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.