A thermometer is a device used to measure temperature that works by detecting changes in a sensor as temperature varies. A mercury-in-glass thermometer was invented by Daniel Gabriel Fahrenheit and consists of a bulb reservoir containing mercury whose volume changes with temperature, and a stem containing a measuring scale. Mercury is well-suited for thermometers as it easily changes between liquid and solid and has a low freezing point. While inexpensive, accurate, and easy to use, mercury thermometers are also fragile, toxic if broken, and pose environmental hazards.