A thermometer is a device used for measuring temperature. This ice-covered thermometer shows that the temperature is about 0 degrees Celsius, or 32 degrees Fahrenheit. The word thermometer comes from the Latin thermo-, meaning ‘heat,’ and -metrum, meaning ‘measure.’ So, a thermometer is a scientific tool for measuring temperature. Thermometers work by utilizing the physical properties of various substances. For example, liquids and solids expand as they heat up and contract when they cool down. Similarly, gases increase in pressure as they heat up and decrease as they cool down. The mercury-in-glass thermometer was the very first, invented by Daniel Gabriel Fahrenheit in 1714. A thermometer contains two essential elements: a sensor that detects a change in temperature and a device that communicates that temperature to the person reading it.