How does a thermostat know the temperature?

A thermostat knows the temperature by sensing how warm the room is and comparing that to the number you chose. Older ones use a bending strip made from two metals; newer ones use electronic sensors. When the room is too cool or too warm, it signals the heating or cooling system.

That old metal-strip design works because different metals grow and shrink by slightly different amounts as temperature changes. One strip can bend enough to touch or leave an electrical contact, so your wall box can switch a furnace on and off.