Cold-blooded describes animals whose body temperature changes with their surroundings, rather than staying constant like ours does. Snakes, lizards, turtles, and frogs are cold-blooded. On a chilly morning, a lizard moves slowly because its body is cool. After warming up in the sun for a while, that same lizard becomes quick and active. Cold-blooded animals don't generate their own body heat the way mammals and birds do, so they rely on external heat sources like sunlight or warm rocks.

Scientists use the term ectothermic to describe these animals more precisely, since “cold-blooded” can be misleading: these animals aren't actually cold. A snake basking on a sun-warmed boulder might have a body temperature of 95 degrees, warmer than a typical classroom.

The phrase also describes someone who acts cruelly without feeling or remorse. A cold-blooded person might do something harmful without showing normal human emotion or sympathy. When we say someone did something in cold blood, we mean they did it deliberately and calmly, not in the heat of the moment. This figurative meaning suggests an unsettling absence of the warmth and feeling we expect from people.