Why did Sally fall off the swing?
She didn't have any 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, ya see? And in the knock-knock part, Sally can't be at the door 'cause, well, she still ain't got no arms! It's funny 'cause it's sad, like your dating life.