A mammal is an animal

What is a mammal? And what is not a mammal? Mammals have skeletons as deer have, breathe air in lungs as whales do, and are born alive as are calves. What is not a mammal? A ladybug has no skeleton, a fish breathes through gills and a bird hatches from an egg. Monkeys, dolphins, and elephants are mammals and so are you and I!