How does a thermostat know the temperature?

A thermostat knows the room's temperature with a sensor: older ones bend a strip made from two metals, while newer ones read heat with electronics. As the room warms or cools, the sensor changes, and the thermostat compares that reading with the number you chose, then starts or stops the heat.

The old metal-strip design works because different metals swell by different amounts when they get warm. A dial does not measure heat by itself; it simply changes how far the moving tip must travel before it closes a circuit, making the heater take turns with the room.