A thermometer is any thermal sensor that measures temperature.
Thermal sensors are materials that change when they are put into air having different temperatures.
The little lines on a thermometer are called calibrations. A thermometer can be calibrated in either Fahrenheit or Celsius scale. A thermometer really measures its own temperature because it becomes the temperature of the air around it. Meteorologists use different kinds of thermometers to measure the air near the earth and far above the earth. It is made of a glass tube with a bulb on one end.