Very Bad Humor

Three Native American women are sitting side by side. The first, sitting on a goatskin, has a son who weighs 170 pounds. The second, sitting on a deerskin, has a son who weights 130 pounds. The third, sitting on a hippopotamus hide, herself weighs 300 pounds.

What famous mathematical theorem does this illustrate?

Naturally, the answer is that the squaw on the hippopotamus is equal to the sons of the squaws on the other two hides.