A thermometer is the instrument used to measure temperature. It operates on various principles, such as the expansion of liquids (like mercury or alcohol) or the resistance change in metals or semiconductors. When measuring temperature, a thermometer is placed in the environment or substance being evaluated, and it displays the corresponding temperature on a scale, typically in degrees Celsius, Fahrenheit, or Kelvin. Thermometers are essential in various fields, including meteorology, medicine, and cooking, providing crucial data for scientific, health-related, and culinary purposes.