A bimetal (bi means 2 or double) consists of 2 metal strips that are very firmly connected to each other. These 2 strips have different expansion coefficients. When warming up/heating, these strips will bend in relation to each other. The curvature of the metals depends on the temperature. The hotter the metal gets, the more curved it becomes.
A construction can be made in which the bimetal breaks the contact when it becomes warped by (too much) heating. A bimetal is therefore often used as a temperature switch or as a safety switch.
Cast-in bimetallic strips are used in cars, among other things, for pistons, the automatic choke, certain dashboard instruments and thermal switches (e.g. the thermal switch mounted on the radiator, which drives the electric fan at a higher coolant temperature) and in the viscous coupling of the engine cooling fan.