A thermometer is used to measure the temperature of different materials or substances. It can be used for solids, liquids or even gases.
A typical thermometer contains a liquid solution inside, which is often mercury. Due to safety concerns over mercury, other solutions like alcohol mixtures are often used instead. As the temperature increases, the liquid expands, while as the temperature decreases, the liquid decreases in volume. The change in volume of the liquid corresponds to a scale that is written on the outside of the thermometer. It is calibrated so that a person can read off the current temperature based on the location of the top of the liquid.