The red liquid inside of a thermometer is alcohol. When the temperature changes, the alcohol expands, providing a reading based on the thermometer's scale. Red dye is used to color the clear alcohol so that someone is able to easily read the thermometer.
Alcohol thermometers are considered much safer than the once popular mercury thermometers, but they are considered slightly less durable because the alcohol evaporates from the thermometer over time.
The mercury in mercury thermometers is poisonous if the thermometer breaks. Old thermometers must be properly disposed of to prevent environmental contamination. The Environmental Protection Agency (EPA) recommends that the use of mercury thermometers be avoided and that existing mercury thermometers be properly retired.