Milk

Milk was a substance secreted by mammals to feed their young.

Milk-producing species
Whether the female of a species exudes milk from its nipples or from simple ducts to nurture its young was one of the defining traits of mammals. (EDA: The Year of Intelligent Tigers) Similar nourishing substances are, however, produced by non-mammalian species. Malmooth could drink their own internal milk, (DW: Utopia) while the green milk produced by Skarasens was fed on by the Zygons. (NSA: Sting of the Zygons, EDA: The Bodysnatchers)

The only known cure for Spectrox toxaemia is the milk from a Queen Spectrox Bat. (DW: The Caves of Androzani)

Milk as a beverage
Milk was also used by other species as a drink. The Humans of Earth harvested and drank the milk of cows.

Milk was often used in tea and coffee to add taste. (NA: Theatre of War) Milk can be sold in cartons or glass bottles, and can be delivered by milkmen. (DW: The Stolen Earth) Milk was sometimes consumed while warm as a sleep aid. (DW: Invasion of the Dinosaurs)

In some locations, such as the planet Sto, milk was sold in a Milk Market. (DW: Voyage of the Damned)

Minor references

 * The Doctor bought some milk. (DWBIT: Wrath of the Warrior)
 * The Doctor later tried to get some more milk, but instead ended up at the Whispering Gallery. (IDW: The Whispering Gallery)
 * Larry Nightingale went out to buy milk. (DW: Blink)
 * Rose commented that they returned to Earth whenever they needed milk. The Doctor questioned why, out of all the species in all the universe, it had to come out of a cow. (DW The Empty Child)
 * Rory Williams was buying milk when he came across an alien. (DWAM: Rory's Story)