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 in Celsius or Fahrenheit. There are various types of thermometers, including digital and infrared models. Digital thermometers provide quick readings and are commonly used in homes and medical settings. Infrared thermometers measure temperature from a distance, making them useful for checking the temperature of surfaces or people without direct contact.