Now the first part of this system is an electronic thermostat which monitors the temperature inside your freezer—usually the refrigerator, too. And I know, that's a thermometer. But I don't know how to draw a thermostat. There. Now this thermostat is connected via electronic wires to a device called a compressor, which is usually on the bottom or the top of the unit. And it's just a small, little device. Usually looks kind of like a tank. And it compresses what is called a refrigerant, usually R134A or something like that. Systems used to use ammonia, which is very, very dangerous. Anyway, it compresses it until it becomes extremely hot, and condenses into a liquid form. It's usually a gas in nature.