HOW IT WORKS: Thermostat
The thermostat regulates the amount of water that goes through the radiator. It allows the engine to heat up quickly by blocking the coolant flow and allowing the coolant  to recirculated back through the engine.

A small cylinder located on the center of the device is filled with a wax that begins to melt at around 180 F (different thermostats open at different temperatures). A plunger (rod) connected to the valve presses into this wax. When the wax melts, it expands, pushing the plunger out of the cylinder and opening the valve. When the wax solidifies the plunger is push back in from the spring. The wax just expands a good bit more because it is changing from a solid to a liquid in addition to expanding from the heat.