Mammals are animals . We need to do better than that. Mammals have a back-bone.
Mammals are warm-blooded.
Fortunately, scientists have already worked it all out for us. This milk is produced by modified sweat glands called 'mammary' glands. It is from these glands that the whole group takes its name, 'Mammals. The second test is the possession of hair, something humans often have problems with but which they should respect more. No other animal has hair in the same form as mammals, and all mammals have some hair at least at the beginning of their lives - baby whales and dolphins are born with a moustache.