You should know better by now. Never make jokes, because then you might be forced to explain them, or justify how realistic the joke is, or maybe ensure that you have properly covered all possible antecedents for the joke!
Olaf the Brave: Knock knock!
Fizzbang the Fantabulous: Who's there?
Olaf the Brave: Cows says.
Fizzbang the Fantabulous: Cows says who?
Olaf the Brave: No, a cow says MOO!
Random internet commenter: I object. Not only do cows not speak, it is completely unclear if there is a door, or if, in fact, bovines exist in fantasy worlds such as the one that Fizzbang and Olaf live in. I demand further explication and debate regarding jokes and the application to the price of tea in China.