Three vampire bats live in a cave surrounded by three castles. One night, the bats bet on who can drink the most blood.

The first bat comes home with blood dripping off his fangs. He says, "See that castle over there? I drank the blood of three people."

The second bat returns with blood around his mouth. He says, "See that castle over there? I drank the blood of five people."

The third bat comes back covered in blood. He says, "See that castle over there?" The other bats nod. "Well," says the third bat, "I didn't."