Bob has no arms.
Knock, knock.
"Who's there?"
"It's not Bob."
The Armless Bob
Categories
Explanation
Experiment
Explain Bear
Listen here, bud. This "Bob" guy doesn't have any arms, right? So when someone says "Knock, knock," it's usually followed by someone SAYING who's there. But ol' Bob can't say who's there, 'cause he ain't got no arms to knock! You probably don't have arms either, considering how dumb this is to you.