Thermometers are devices used to measure temperature of a body or substance. Thermometers, typically, consist of a material that responds to change in body's (or substance's) temperature and a mechanism to show that change. The material chosen for thermometers should respond quickly to temperature changes, that is, it should heat or cool rapidly when in contact with a hot or cold body, respectively. This will enable rapid measurement of substance's or body's temperature. The heating or cooling of this material must be totally reversible, that is, that material should heat up when in contact with a hot body, but should come back to its original temperature when not in contact with the hot body. This allows repeated use of the thermometers. Also, the temperature volume relationship should be monotonic, that is over a given range of temperatures, the volume should either monotonically increase or decrease (but not both) as the temperature increases and this should be true for all the temperatures in the range. Furthermore, the volumetric expansion should have upper and lower bounds, that is, the material should expand (or contract) on temperature changes but the change must be appreciable (to be observed by naked eyes, even at smaller temperature changes), but should be small enough to allow for large temperature measurement range in a compact sized thermometer. Mercury is the commonly used material for thermometers.
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