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. In this short film we follow human geneticist Spencer Wells, Director of the Genographic Project of the National Geographic Society, as he tracks down the genetic changes associated with the ability to digest lactose as adults, tracing the origin of the trait to less than 10,000 thousand years ago, a time when some human populations started domesticating animals, including goats, sheep, and cows. Combining genetics, chemistry, and anthropology, this story provides a compelling example of the co-evolution of human genes and human culture.