Knock-knock jokes are often the first type of joke that children learn. They're formulaic, interactive, and have delightfully punny punch lines. But humour is a tricky concept, and just because a kid's memorized the formula doesn't mean they understand the equation. Here are some I've been subjected to: Kid: Knock, knock! Me: Who's there? Kid: Moo! Me: Moo, who? Kid: A cow Knock, knock! Who's there? Me. Me, who? Is Me, Mary!! Who's there? Isabelle. Isabelle, who? I gots a bicycle! Who's there? Dick. (Hmmm. Do I want to pursue this one? Ah, why not: the tot won't understand the punch line anyway!) Dick, who? I dunno. Daddy wouldn't tell me the rest. Who's there? Banana. (I heave a mental sigh. I know this one, and it can go on for a while in the hands of a knock-knock fanatic.) Banana who? Banana Banana who? Banana Banana who? Orange. Orange who? Banana went away. (For those less familiar with these things, we've omitted a critical part of the punch line: Orange you glad...) Who's there? Amos. Amos, who? A mosquito just bit me! Hey! Someone got one right!