Which of these is NOT a mammal?

The crocodile is a reptile - scales rather than hair, eggs rather than live birth, and no milk. The mammal test is really just those three: hair at some point in life, milk, and three small bones in the middle ear.

The edges of the category are stranger than the middle. Monotremes - the platypus and the echidnas - are mammals that lay eggs, and they have no nipples; the milk seeps through patches of skin and the young lap it from the fur. Whales and dolphins are mammals with no visible hair at all as adults, though they carry a few bristles as embryos, and bats are mammals that fly.

Crocodiles are also not the primitive leftovers they are usually taken for. They have a four-chambered heart like a bird or a mammal, an unusually sophisticated one, and they care for their young - carrying hatchlings to water in their jaws and guarding them for months. Their closest living relatives are not lizards or snakes but birds, and the two together are what remains of the archosaurs, the group that included the dinosaurs.