Temperature is known to most people as either hot, cold, or somewhere in between. For scientists, temperature is really the measure of the average energy of the molecules of a liquid, solid, or gas. The higher the temperature, the faster the molecules move. The fast-moving molecules slam into your skin and transfer some of their energy to you in the form of heat. The transfer of energy moves heat from hot surfaces to cold surfaces.
So how does a thermometer work? Well, the faster the molecules of alcohol (or mercury) move in the thermometer, the more space, or volume they take up. The more volume the molecules take up, the higher the alcohol moves up the stem of the thermometer from the bulb of the thermometer. This phenomenon (when observed in a gas) can be described by the ideal gas law, Pv=nRT. Pressure is proportional to the temperature; as the temperature goes up, so does the volume of the gas.

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