Tell students that this picture shows two thermometers that are identical in every way, except one has alcohol and the other has mercury inside. Point out that both thermometers are placed in hot ...

The thermometer has a small bulb filled with either mercury (in older thermometers) or alcohol (in modern ones). When the temperature rises, the liquid expands and moves up the narrow tube.

The mercury-in-glass thermometer was used for almost a century ... fast and convenient, didn't require alcohol soaking and cleaning, or pose the risk for breakage and mercury exposure.