The kind of milk sold in stores mostly comes from dairy cows. All mammals produce milk to feed their young, but only certain kinds of animals are farmed for their milk by humans.
Mammals are a group of creatures distinguished in part by their mammary glands. Mammary glands are modified sweat glands that produce a nutritious liquid that helps very young mammals grow and thrive. On humans, the structures associated with the mammary glands are called breasts. On cows and goats, another animal that is farmed for milk, they are called udders. Other animals whose milk is utilized by humans include buffalo, sheep, camels, yaks, donkeys, horses and reindeer. Buffalo are the second-most popular milk-producing species after cattle.