A thermostat is a device that automatically controls temperature by turning heating or cooling systems on and off. You'll find thermostats on the walls of most homes, usually looking like a small rectangular box with buttons or a dial and a display showing the current temperature.

Here's how it works: You set the thermostat to your desired temperature, say 68 degrees. When the room gets colder than 68, the thermostat signals the furnace to turn on and warm things up. Once the temperature reaches 68, it tells the furnace to shut off. In summer, the same device controls air conditioning, keeping your home from getting too hot.

Modern thermostats can be programmed to change temperatures at different times of day, so your house warms up before you wake up and cools down while you're at school, saving energy.

You'll also find thermostats in car engines, refrigerators, ovens, and even coffee makers. Anywhere something needs to stay at a steady temperature, a thermostat is probably doing the work behind the scenes, constantly measuring and adjusting without anyone having to think about it.