From Wikistix

Three Red Indian squaws slept on three different kinds of animal skins. One slept on an elk skin, another on a buffalo skin, and the third on a hippopotamus skin.

Later they all had children - the first squaw a daughter, the second a son, and the third twins, a daughter and a son. Proving that the squaw on the hippopotamus is equal to the sum of the squaws on the other two hides.