Monkey business has taken on a whole new meaning. Sex between marmosets, small South American monkeys, is more mischievous and deceitful than either of the "parents" are aware. If you read the science sections in the paper you'll know why. A study released in this week's Proceedings of the National Academyof Sciences tells the story. Marmosets often give birth to fraternal twins. Fraternal twins develop from two eggs and have different DNA. When marmoset twins are in the womb they exchange cells--an occurrence that is rare in the animal kingdom, but common in marmosets. The monkeys trade cells for everything from hair to sperm. Which means that a male marmoset, unbeknownst to him, may be spreading the seed of his brother. The new "father" will actually be the monkey's uncle. The scientific press has pounced on the story with a abundance of no-so-catchy phrases and puns. Read enough of them and you'll eventually end up saying "uncle."