The thermostat is made based on the principle of thermal expansion and contraction of objects. Thermal expansion and contraction is a common property of objects, but different objects have varying degrees of expansion and contraction. The two sides of the bimetallic strip are conductors made of different materials. Under changing temperatures, the strip bends due to the differing rates of expansion and contraction, and when it contacts a preset contact or switch, it activates the designed circuit (protection). 
     This is the basic working principle and design concept of the thermostat.