Human babies drink milk; it's the food especially provided for them by their mothers. Various cultures have also added the milk of other mammals to their diet, and adults think nothing of downing a glass of cows' milk. But worldwide, only a third of adults can actually digest lactose, the sugar in milk. Human geneticist Spencer Wells tracks down the genetic changes associated with the ability to digest lactose as adults. Combining genetics, chemistry, and anthropology, this story provides a compelling example of the co-evolution of human genes and human culture.