Pat Cristo, age 15, of Omaha, Nebraska, for her question:

What is meant by absolute zero?

Heat is a form of energy. It is the energy that atoms and molecules use to move faster. The faster they go, the hotter a substance becomes. As they slow down, a substance loses heat and becomes cooler. This, we are told, is what happens in the invisibly small world of molecules. From this idea we can deduce that there must be a limit to the amount of heat energy a substance can lose.

This limit is rated as absolute zero and given a different number on the different temperature scales. A substance is said to reach absolute zero when it loses the last fraction of its last degree of temperature. At this point, its atoms and molecules are incapable of making the slightest shudder or vibration. This impossible colder than cold temperature is reached at minus 459.67 degrees Fahrenheit, which is the same as minus 273.15 degrees centrigrade or zero degrees K (Kelvin scales). Nothing can get colder than that.