[Jokes] Puzzle

[Justin Mitchell]

Three Red Indian squaws get pregnant around the same time. They all
follow the custom of squatting on animal skins during preganancy; two
choose Buffalo, the third, Hippo (they're a well-travelled tribe).

After nine months, the first two give birth to sons. The third stays
heavily pregnant, but nothing happens. After eighteen years, the two
boys have grown into fine braves, but the third mother is still
squatting. The tribe believe that she is obviously a witch, and they
charge the two braves with dispatching her in the ritual style.

They enter her tent at dead of night. Horrible cries are heard, and
sounds of violence ring out for several minutes. When the braves fail to
emerge, the chief enters the tipi to find the pregnant woman, still
squatting, and the two warriors lying unconscious on the ground. He
decrees that, however strange she may seem, the witch-woman must be left
alone and forever respected, for she has taught them an immutable law of
the universe.

What is that law?