The main characteristic of a mammal is the presence of mammary glands, which produce milk to nourish young offspring. This trait is key to defining mammals and is accompanied by other features such as fur or hair on the body, three middle ear bones, and a backbone. Additionally, most mammals give live birth, with exceptions like monotremes (e.g., the platypus and echidna) that lay eggs. Mammals are also warm-blooded creatures capable of regulating their body temperature independently of the environment.