What does he mean by "a knock-knock joke"?
Native speaker’s answer
Rebecca
A "knock-knock joke" is a classic and typical form of a joke. These jokes are popular among children. It requires participation from another person or an audience and typically ends with a pun! The joke starts with "knock-knock", and the other person responds with "who's there?". Here are a couple of examples: A: Knock-knock! B: Who's there? A: Broken pencil. B: Broken pencil, who? A: Nevermind. It's pointless. => punchline. A: Knock-knock! B: Who's there? A: Boo. B: Boo, who? A: Don't cry. It's only a joke. => punchline.