A mammal is a warm-blooded animal that feeds its babies with milk produced by the mother's body. You're a mammal. So are dogs, cats, horses, whales, bats, and elephants. In fact, there are more than 6,000 different species of mammals living on Earth today.

What makes mammals special? Besides producing milk, mammals have hair or fur (even whales have some), and most give birth to live babies instead of laying eggs. Their warm blood means they can keep their body temperature steady whether it's freezing cold or blazing hot outside, unlike reptiles, which need to sun themselves to warm up.

Mammals come in astounding variety. The blue whale is the largest mammal, weighing as much as 200 tons. The Etruscan shrew is one of the smallest, weighing less than a dime. Some mammals, like dolphins and bats, use echolocation (bouncing sound waves off objects) to navigate in the dark. Others, like cheetahs, are built for incredible speed.