Mercury is sealed in a glass tube and graduation marks are etched in the glass. The etch marks are spaced in such a way as to represent a certain amount (degree) of rise/fall. As the temperature changes outside the tube, the level of mercury inside the tube rises and falls in proportion to the change in temperature. BTW: The glass tube thermometer was invented by a fellow named Fahrenheit.

 If one breaks, Please don't touch the mercury. Instead, leave the room for a while so the fumes dissipate. Then use gloves and a scoop to pick it up and put in a container to dispose with toxic chemicals.

I often wonder how many of today's medical problems are caused because we, children before the 1970's played with mercury because it was such a strange substance. I think some kids even broke the thermometer on purpose.

I still have one of those thermometer from those childhood years of long long ago.