Thermometer is an instrument that measures the temperature of gases, liquids, and solids. As the temperature of a substance changes, some of its chemical and physical properties also change. For example, substances may change in color, shape, or volume. A thermometer measures one of these changes and usually displays the size of the change as a number. This number can be converted to a temperature value by using a suitable temperature scale. Such scales include the Fahrenheit, Celsius, and Kelvin scales.