Craig Maringer, age 14, of Bessemer, Ala., for his question:

HOW DOES A THERMOSTAT WORK?

A thermostat is a device that automatically regulates the temperature of a system by maintaining it constant or varying it over a specific range. You'll find thermostats in industrial furnaces, in heating systems and in other engineering applications.

Most thermostats depend on the expansion of a substance corresponding to an increase in temperature. In the water type thermostat, for example, numerous flat disk shaped shells, called sylphones, are either filled with gas or partially filled with a liquid that boils at the desired