The primary characteristic of a mammal is the presence of mammary glands, which allow females to produce milk to feed their young. This feature, along with having hair or fur on their bodies, differentiates mammals from other animal classes. Mammals are warm-blooded and typically have more complex brain structures, enabling advanced behaviors and social structures. They also give live birth, with the exception of monotremes like the platypus and echidna, which lay eggs. Mammals are found in a diverse range of habitats across the planet, showcasing remarkable adaptations that enhance their survival.