Thermostats are used to regulate temperature by controlling gas flow to burners. There are four main types of thermostats: rod and tube, liquid expansion, vapour pressure, and bimetal. Rod and tube thermostats use materials with different coefficients of expansion. Liquid expansion thermostats use the expansion and contraction of liquid to open and close a bellows. Vapour pressure thermostats use changes in gas pressure from temperature changes. Bimetal thermostats bend from differential expansion of bonded metals. Thermostats can function by snapping fully on/off or modulating gas flow to maintain temperature.