A thermometer is a device used to measure temperature. It typically consists of a glass tube filled with a liquid, such as mercury or colored alcohol, which expands or contracts as the temperature changes. The scale on the thermometer indicates the temperature in degrees, often using the Celsius or Fahrenheit systems. Thermometers come in various types, including digital and infrared models. Digital thermometers provide quick readings and are commonly used in medical settings to check body temperature. Infrared thermometers measure temperature from a distance, making them useful for checking the temperature of surfaces or objects without direct contact.