This document discusses how animals control their body temperature. It notes that ectotherms rely on the environment to control their temperature while endotherms generate their own body heat. It emphasizes that controlling body temperature is important for homeostasis, especially in habitats with wide temperature variations that change with the time of day or seasons. Most reptiles, fish and amphibians are ectotherms whose temperatures depend mainly on gaining or losing heat to their surroundings.