Knock knock. Who's there? Lettuce.

This absolute classic is a masterclass in the art of the pun. The humor hinges on a simple but brilliant bit of auditory trickery, using the name of a common salad green as a phonetic bait-and-switch. The entire setup is designed to fool the listener into saying the crucial phrase "Lettuce who?", which sounds identical to "Let us who?". This perfectly tees up the punchline, transforming a vegetable's name into a desperate plea for entry. It’s a perfect example of how the knock-knock format uses misdirection (Review) to land a laugh.

As one of the most famous knock-knock jokes of all time, this one is a hall-of-famer in the world of kid-friendly humor. It's often one of the first jokes a person learns, right up there with its fruity cousins. Part of its enduring charm is the slightly surreal mental image it creates: a poor, shivering head of romaine or iceberg, knocking politely at a door, just wanting to get out of the cold. Who could be so heartless as to say no?