Beg to differ Hagar, but I think they're more an on/off device. The bulb in the thermostat is filled with a sort of wax that changes from solid to liquid at a certain temperature. This phase change corresponds to a volume change that actuates the thermostat. However, it does take a certain time for the phase change to take place over the full volume of wax, so in the short term the thermostat may be happy staying half open. But I agree that they don't just snap open/closed!