Temperature is a physical quantity which gives us an idea of how hot or cold an object is. The temperature of an object depends on how fast the atoms and molecules which make up the object can shake, or oscillate. As an object is cooled, the oscillations of its atoms and molecules slow down. For example, as water cools, the slowing oscillations of the molecules allow the water to freeze into ice. In all materials, a point is eventually reached at which all oscillations are the slowest they can possibly be. The temperature which corresponds to this point is called absolute zero. Note that the oscillations never come to a complete stop, even at absolute zero.

There are three temperature scales. Most people are familiar with either the Fahrenheit or the Celsius scales, with temperatures measured in degrees Fahrenheit (º F) or degrees Celsius (º C) respectively. On the Fahrenheit scale, water freezes at a temperature of 32º Fahrenheit and boils at 212º F. Absolute zero on this scale is not at 0º Fahrenheit, but rather at -459º Fahrenheit. The Celsius scale sets the freezing point of water at 0º Celsius and the boiling point at 100º Celsius. On the Celsius scale, absolute zero corresponds to a temperature of -273º Celsius.

Scientists - especially those who study what happens to things when they become very, very cold - commonly use the Kelvin scale, with temperatures measured in Kelvin (K). This scale uses the same temperature steps as the Celsius scale, but is shifted downward. On this scale, water freezes at 273 K and boils at 373 K. Only on the Kelvin temperature scale does absolute zero actually fall at 0 K.  The accompanying diagram compares the three temperatures scales.

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