how do thermostats work?

Answer 1

Assuming you are referring to an automotive coolant thermostat---
The device is placed in the coolant flow stream, and restricts the flow of coolant to control OVER cooling. There is a temperature sensitive bellows that is in contact with the coolant.
When the coolant reaches a predetermined temperature, the bellows expand, and moves the orifice pintle off the seat, allowing the coolant to flow, and give up its heat through the radiator.
When the coolant reaches a cooler level, the bellows closes and the pintle seats again restricting the flow of coolant. This cycling continues non-stop to maintain proper coolant temperature.
A bit over-simplified, but a clear summary of operation.

Good Luck

Answer 2

Try this site first and read.

http://www.howstuffworks.com/home-thermostat.htm

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