* Mercury is a liquid metal that expands and contracts predictably with temperature changes. It was the standard for many years due to its high thermal conductivity and visibility. However, mercury is toxic, so its use has been phased out in many applications.

* Alcohol (usually colored red or blue for visibility) is another common substance used in thermometers. It is less toxic than mercury and expands and contracts at a more predictable rate than mercury, making it more accurate for measuring temperature changes.

While there are also digital thermometers, the traditional thermometers used for room temperature often utilize these materials.