A radiator is a device that transfers heat from hot water or steam into the air to warm a room. In many homes and apartments, radiators are those ribbed metal units along the walls that get hot during winter. Hot water or steam flows through pipes inside the radiator, and the metal surface releases that heat into the surrounding air, gradually warming the entire room.

Old-fashioned radiators often make clanking or hissing sounds as the hot water or steam moves through them, and people sometimes drape wet mittens or clothes over them to dry.

Cars also have radiators, but they work in reverse: instead of warming a room, a car's radiator cools the engine by releasing excess heat into the air. As the engine runs, it generates tremendous heat that could damage its parts. Coolant liquid circulates through the engine, absorbs that heat, then flows to the radiator, where the heat escapes into the air flowing past the car. Without a working radiator, a car's engine would overheat within minutes.