Liquid thermometers contain either mercury, alcohol or mineral spirits. Mercury thermometers, which are not widely used any longer as of 2015 due to the toxic nature of the element, have a silver liquid in them. Alcohol and mineral-spirit thermometers have a dyed liquid, as the liquids are naturally colorless.
The use of mercury in thermometers dates back hundreds of years. It was widely used because it reliably expands and contracts with changes in temperatures. It is highly poisonous, however, and can enter the body through the respiratory or digestive tracts, or directly through the skin. The use of spirit-based liquid thermometers, as well as non-liquid digital, dot-matrix and indium-gallium-tin alloy devices, represents a concerted effort by the EPA to remove mercury from the cycle of consumption.