Ever heard of a polar bear plunge? During winter, people jump into the ocean to raise money for a charity. They don’t stay in there long, and if they did, they’d definitely get hypothermia because the water’s so cold. The word is a combination of the prefix hypo-, meaning “below,” and the Greek word thermē, translated as “heat.” When your body heat gets below what it should be, please put on a sweater or you’ll get hypothermia and maybe die.