What is a Mammal?

What makes a mammal a mammal? How can creatures as different as the bat, the tiger, and the whale all belong to the same class of animals? Are mammals creatures that bear live young? Most are, but the duck-billed platypus and some spiny anteaters lay eggs, and they're mammals. Do mammals live on land? Many do, but dolphins and whales live in the sea, and they're mammals, too. So what exactly is a mammal? Answering that question isn't as easy as it might seem, but What is a Mammal? makes solving the puzzle fascinating and fun.