Just because we can't see air and so often overlook its presence doesn't mean the space around us is empty. Like all substances, air is made up of atoms. Among the elements that constitute air are nitrogen, oxygen, carbon, and hydrogen. In air, however, these atoms and their combinations of molecules are spread out more widely than the atoms and molecules that make up liquids and solids.

Somewhat surprisingly, air even has weight. In huge volumes, such as the column of air that makes up the atmosphere above our heads, air exerts tremendous pressure. At sea level, for example, air pressure is equivalent to 14.7 pounds of weight on each square inch of surface. Yet we pay no attention to this pressure because we, along with most other forms of life, evolved under these conditions. Indeed, if air pressure were to suddenly disappear, we wouldn't survive its absence.

If we pay attention, we can see and hear signs of the air around us. Riding a bicycle quickly, we can feel it moving over our skin as we, in turn, move through it. In fact, air resistance is one of the most important factors limiting the speed of all kinds of vehicles, from bicycles to jet airplanes. Pushing against air requires a tremendous amount of energy -- and air resistance only increases as speed does.

Moving air can also provide useful energy. It can fill the sails of a sailboat and push the boat forward without the use of any kind of motor. In consistently windy locations, air can be used to turn windmills and generate electricity for human use. Air is an extremely attractive energy source -- after all, it is entirely nonpolluting and endlessly renewable. Unfortunately, it's not practical in locations where wind doesn't blow regularly. Although the air itself never disappears, it only provides power when it's moving as wind.