One known exception is Koko, the famous western lowland gorilla who understands more than 1,000 American Sign Language signs and 2,000 spoken English words. The clever primate is known not only to use language to humorous effect by playing with different meanings of the same word, but also to understand slapstick comedy — she’s reportedly signed the word “chase” after tying her trainer’s shoelaces together and made laughing noises at her trainer’s clumsiness.

But there are various issues with the incongruity theory of humor and related theories, including the fact that they can’t explain how jokes with predictable punch lines are funny and how certain incongruous things aren’t funny. They also struggle to account for a prime reason we laugh: Being tickled by a friend or family member.