Video narration
What makes a thermostat more than a command? Send a command without checking, and you cannot tell if the room reached your target. That setting is the setpoint. A sensor measures the room; the difference between desired and measured temperature is the error. The controller uses that error to command the actuator, the heater, which changes the plant—the room. Set the thermostat higher than room temperature. The sensor reports the lower temperature, creating an error. The controller calls for heating, and the heater warms the room. Then the sensor measures again, feeding it back into the comparison. That is the key flip: the output is measured, so the system corrects itself instead of acting blindly. This is closed-loop control: negative feedback reduces the error. So what makes the thermostat more than a command? It keeps the room on target by measuring, comparing, and correcting.