Warm-blooded animals maintain a steady internal body temperature regardless of their surroundings. Whether it's freezing outside or blazing hot, a warm-blooded animal's body stays at roughly the same temperature. Mammals (like humans, dogs, and whales) and birds are warm-blooded, which scientists call endothermic. A human body temperature stays around 98.6°F whether someone is building a snowman in winter or swimming at the beach in summer.

This ability comes at a cost: warm-blooded animals need lots of fuel. They eat a lot of food because their bodies burn that food to generate heat and keep their temperature stable. A lion might eat a huge meal and then rest for days, but it still needs far more food over time than a cold-blooded lizard of similar size.

The opposite is cold-blooded, like reptiles, fish, and insects. These animals' body temperatures change with their environment. A snake might bask on a warm rock to heat up or hide in a cool burrow when it gets too hot. Cold-blooded animals need less food, but they can't stay active when it gets cold, which is why you don't see many snakes slithering around in winter.

The term “warm-blooded” can also describe someone with strong feelings or passion, though this meaning is less common.