How do humans keep warm?

Warm-blooded creatures, like mammals and birds, try to keep the inside of their bodies at a constant temperature. They do this by generating their own heat when they are in a cooler environment, and by cooling themselves when they are in a hotter environment. To generate heat, warmblooded animals convert the food that they eat into energy. They have to eat a lot of food, compared with cold-blooded animals, to maintain a constant body temperature. Only a small amount of the food that a warmblooded animal eats is converted into body mass. The rest is used to fuel a constant body temperature.

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They also cover themselves in insulated, warm clothing and heat their dwellings with fire.