Many hot things take up more room than the same things when they are cold. Thermometers take advantage of that to measure heat. The old-fashioned kind of thermometer you see here is a narrow glass tube with a glass ball or bulb at the bottom. Inside the glass is a little bit of mercury or another liquid (it is not mercury in this picture). When the liquid is cold, the molecules stay quiet and close together, and they don't take up much space, so they can all fit inside the bulb down at the bottom. But as the liquid gets hotter, for instance if you rub the thermometer, or put it in your mouth, or if the sun is warming up your part of the earth, the molecules of liquid begin to move around more and bounce off each other. Then they take up more room. They don't fit in the bulb anymore, so they have to travel up the tube. The hotter it gets, the higher the liquid goes up the tube.
The liquid doesn't care what is written on the outside of the tube; those numbers are just so we can measure how far up the liquid has gone. In the United States, people often measure heat in Fahrenheit degrees. Everywhere else in the world, people measure heat in Celsius or Centigrade degrees, which are the same thing. Celsius degrees are bigger than Fahrenheit degrees. On the Celsius scale, water freezes into ice at zero degrees and boils at 100 degrees. On the Fahrenheit scale, water freezes at 32 degrees and boils into steam at 212 degrees. So from freezing to boiling there are 100 Celsius degrees, but there are 180 Fahrenheit degrees. It's about 100 degrees Fahrenheit on the thermometer in the picture - what is the temperature in Celsius degrees?
To find out more about temperature, check out these books from Amazon.com or from your library: