Me: Y’know, they’re always talking about cow milk and goat milk. Nobody ever talks about chicken milk.
Wife: I don’t think you can get milk from a chicken. Can you?
Me: No. I was just joking. You have to be a mammal to have milk: There’s a link between the words “mammal” and “mammary”.
Wife: Really? Hmm. Then what’s a chicken?
Me: A chicken is a bird.