All mammals have hair that covers at least part of their bodies, nurse their young through mammary glands and have muscular, four-chambered hearts. All mammals also have three inner ear bones: the incus, malleus and stapes, and lower jaws made of a single bone. All mammals are vertebrates, which means that they have backbones, and they are all warm-blooded, which means they are capable of regulating their own body temperature.
The fact that mammals are warm-blooded makes them able to live in most any climate on Earth. Their bodies are capable of maintaining a constant internal temperature regardless of their surroundings. Maintaining a constant body temperature requires a lot of energy, so mammals tend to eat plentiful diets.
Female mammals produce milk for their young, and this milk is secreted through mammary glands. Most, but not all, mammals have mammary glands that are encased in nipples. The platypus and echidna do not have nipples, and instead secrete milk from specialized ducts in their abdomens. Producing milk allows mammals to spend more time with their young and teach them how to survive.
Mammals are the only class of animals on Earth that have hair. Some mammals are completely covered in hair, while others, such as whales, have only sparse patches of hair. Hair helps mammals regulate their body temperature by providing a layer of insulation between their bodies and the external environment.