Water, a substance almost as old as the earth itself, covers about three quarters of the planet's surface. It is a very simple molecule, composed of two hydrogen atoms and one oxygen atom, but vital to life on the planet. Water is found in every cell of every plant and animal in the world, and life could not exist without it.

Water moves through the earth's natural systems in what is called the water cycle. The same water molecule may be in a river one day, the sky the next day, on the tip of a glacier the following day, and eventually released back into the atmosphere when the glacier's tip melts. The water molecule itself never changes; it simply changes its location.

As with sunlight, humans have benefited from the energy made available by the presence and movement of water. Waterwheels, turned by the flow of a...