In a car, the radiator is a crucial component that's necessary to keep the oft-toasty-hot engine cooled and kept within its safe operating temperatures. In essence, a radiator works as a heat exchanger. Once your engine gets hot enough, the thermostat opens, allowing chilled coolant to flow into the engine. This coolant would then absorb the heat from the engine, before then circulating back to the radiator. The radiator has numerous small passageways arranged as fins to help maximize its surface area. There, coolant would flow through those fins, where drag from the cold outside air would gradually cool them down as you're driving along. Then, the whole cycle repeats.