Megan Knight, age 9, of Mundelein, Illinois, for her question:

What are rocks made of?

All the solid rocks in the world are made of tiny atoms. So are all the runny liquids and the filmy gases. As a rule, the atoms are packaged in neat little bundles called molecules. Different substances are made from different atoms and molecules. The world is warm enough to melt most of the water and other liquids. But the rocks need a lot more heat to make them melt. Although they're not cold as ice, actually they are frozen solid.

The atoms in gold are all alike. A few other substances are made from atoms that are all alike. Altogether, the earth has only about 92 different atoms with which to build things    but, assorted atoms can be packaged to make thousands of different molecules. Most of the material in sand is made from molecules of oxygen and silicon. All the various rocks are made from different molecules or mixtures of molecules.