Why did Sally fall off the swing? Because she had no arms.
Knock, knock. "Who's there?" Not Sally.
Sally's Lack of Arms
Explanation
Explain Bear
Listen up, buttercup. Sally fell off the swing because she ain't got no arms, you see? And when someone says "Knock, knock," you ask "Who's there?" But it's not Sally answering 'cause, well, she's armless and can't knock. It's funny 'cause it's terrible. You probably need me to tell you to wipe your own butt, you dumb dumb.