What makes the cooling light go from hot to cold on a cavalier while driving?

What makes the cooling light go from hot to cold on a cavalier while driving?
this isn't unique to just cavaliers there are two main things that can cause this on ANY car. one: you turned on the heater. the heater core works by pulling hot coolant from the engine into a heat-sink type of device that the blower motor then runs across. this acts like a second radiator and when its cold out will actually drop the coolant temp enough to show up sometimes even hot. im a mechanic and i have had at least one car i can remember that was overheating on a long trip so i didn't exactly have access or time to fix it so i drove in the middle of the summer with the heater on and the windows down to keep the engine cool and it actually worked. second if the heater is not on then the thermostat is seizing up and needs to be replaced or the water pump is bad and needs to be replaced.