Milk
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Milk is a nutritious fluid female mammals produce to feed their offspring.
Since the development of agriculture humans have harvested milk from domesticated animals, for human consumption.
Raw milk, usually from cows, but sometimes from goats, camels, horses and buffalos is also harvested, then shipped to a dairy, for pasteurization, or for being turned into milk products like yogurt, cheese, butter.
source
All mammals have Mammary glands - it is the defining feature of a mammal. Except for rare archaic species, like the platypus, all mammals have breasts, with teats, their offspring put in their mouth. Their offspring's mouths stimulation of nerves in the teats release a flow of milk.
Mammals breasts come in pairs. Humans and other apes have a single pair. Animals that give birth to large litters of young, at a single time, have multiple pairs. Marsupials - mammals that nurture their young in a pouch, can have multiple nipples produce milk of different nutritional quality, from different nipples, because they can carry young of varying ages in their pouch.
Composition
In addition to sugars and protein milk is a source of fat. The milk from different mammals contains different levels of milkfat. And the composition of the milk can vary as their young grows older.
Mothers with newborn young produce a special milk, called colostrum, for the first day or so, through which they pass on markers for their offspring's immune system.