The thermostat is in the engine's cooling system, and it works by opening and closing a valve that controls the flow of coolant. When the engine is cold, the thermostat is closed, preventing coolant from flowing through the engine. As the engine warms up, the thermostat opens, allowing coolant to flow through the engine and absorb heat. This helps to bring the engine up to its optimal operating temperature quickly and efficiently. Once the engine reaches its optimal temperature, the thermostat maintains a consistent temperature, which can improve fuel efficiency and reduce emissions.