Almost every device we use has a thermometer in it, even if we don't know it. They are used to determine whether or not different electronics have overheated, or if they are too cold. They work on a different principle than a typical mercury thermometer, using a resistor that is affected more by temperature than a normal resistor. The changes in resistivity are used to judge the temperature, which can be measured very accurately depending on the quality of the resistor, and the supporting electronics. Not only is this useful for preventing damage to electronics because of high temperatures, but it is also useful for making small thermometers that can measure temp from just about anywhere.