Forrest Smith, age 13, of Sioux City, Iowa, for her question:

What is distilled water?

The basic particle of water is a package of one atom of oxygen and two atoms of hydrogen. The average water molecule is an interlocked pair of these basic particles. It is, of course, too small for the human eye to see and it takes billions and billions like it to form a drop of liquid water. And water, in drops or in heaving oceans, is a solvent, a busy dissolves of other chemicals it touches. Mixed among the molecules of ordinary water are assorted chemicals dissolved from the solid rocks, other liquids and gases dissolved from the air. In nature, it is impossible to find supplies of absolutely pure water, unmixed with other chemicals.

It is possible, however, to remove a supply of water molecules from all or most of their invisible fragments of dissolved impurities. The trick is done by distilling ordinary water. When water boils, its molecules separate and go off as individual particles of steamy vapor. The steamy gas leaves most all of its impurities behind. It can be piped off and chilled to make it condense back into liquid form. If the process is done under sterile conditions, this distilled liquid is pure water.