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Body Temperature
Mammals are endothermic, meaning they can regulate their own body temperature. A mammal's metabolism controls the amount of heat its body produces. The sweat glands help cool the body when needed. This allows a mammal's body temperature to remain a constant temperature regardless of its surroundings.
Mammary Glands
A major identifier for mammals is the mammary gland, a modified sweat gland that secretes milk. In fact, this is what gives mammals their name. Mammary glands fully develop in females and are used to feed an animal's young. The milk contains water, carbohydrates (including lactose), protein, minerals, fat and antibodies. The young drink this through nipples or ducts to grow and strengthen.
Hair/Fur
Another major physical characteristic of mammals is hair. All mammals have hair or fur covering at least part of their bodies when they are born, though some lose this hair over time. For example, baby whales and dolphins are born with a moustache. The hair can be for insulation, for example a thick coat of fur. Alternatively, it can be used as a sensory body part, as is the case with whiskers. Or it can be a defensive weapon, like a hedgehog's quills.
Other Characteristics
There are several other characteristics unique to mammals. A mammal's lower jaw is a made up of one bone, while other vertebrates have more than one bone on each side of the jaw. The middle part of a mammal's ear has three bones: the stirrup, hammer and anvil, which used to be part of lower jaw. As mammals evolved, these moved to become part of the ear area. Unlike reptiles and fish, mammals only have two sets of teeth.
Another unique characteristic found only in mammals is the diaphragm, which is a sheet of muscle and tendon that separates the body cavity into two. The lungs and heart is one section, while the liver, kidney, intestines and stomach are in the other. Also, mammals give birth to fully formed young. The young are nurtured inside the female by the placenta, which allows the babies to be nourished without the mother's immune system attacking them.