Mammals are warm blooded; the heat they need to live is made inside their body.
Usually, mammals feed their young with milk they make from their body.
Mammals have what is called hair or blubber, even if there is not much of it.
Mammals give birth to living young (although monotremes, like the platypus, lay eggs).
Mammals' babies grow completely inside its mother's uterus (in marsupials, like the kangaroo, a baby grows mostly in a pocket of skin on the mother's stomach).
