How Does a Thermometer Work?
Most thermometers are sealed glass tubes with liquid inside. A thermometer measures heat by the amount of liquid that expands when it becomes hot and the amount that contracts when it becomes cold. The liquid is either mercury, which is silvery, or alcohol (which is usually tinted red so that it can be seen easily).
Whenever the liquid in a thermometer becomes warm, it expands and rises in the tube. When the temperature becomes cooler, the liquid cools and shrinks back down. (Of course the glass tube also expands but only by about 1/10 as much.) Marks along the side of the thermometer display the temperature in degrees.
In order to measure temperature correctly the thermometer needs to be calibrated by marking it at some known temperatures. For a Celsius thermometer we could mark the thermometer at 0 degrees for the temperature of freezing water and 100 degrees for the temperature of boiling water. Then we could divide up the distance in between the 0 and 100 degree marks into 100 units to measure temperatures in between.