Sally's Fall From The Swing

The joke plays on a morbid twist. First, we're given a setup about Sally falling off a swing, implying a normal accident. Then, the punchline reveals she has no arms, making it absurd. Finally, the knock-knock part uses the setup to confirm Sally is unable to answer the door because she is not alive or present. It's dark humor because it finds amusement in a rather grim situation.