How thermostats work in fire alarms?

A thermostate is a thing which can control or in easy Words we can say it is resistant to heat.

Like we can take the example of thermopole classes ( disposable classes ) if we pore hot tea in it and hold it in our hands so we will not be able to feel the heat as thermopole is heat resistance.

Thermo means heat like you can say thermometer. It is used to know the temperature of heat.

As thermo means heat and meter means measurement