Jeff West, age 9, of Ottaw., Ontario, Canada, for his question:

How do geodes form?

Sometimes you find a rough old rock that has broken loose from a nearby bed of sandstone. If it feels lighter than it should, it may be a geode. But you have to cut that hard, rough stone in half to prove it. Then you see that the inside is lined with colored layers, usually made from hard silica minerals. Sometimes the center of the rocky pocket is almost filled with colored crystals of quartz. The earth was able to create it because moving water dissolves rocky minerals and sometimes has to drop them, molecule by molecule.     

The geode's story began ages ago. Water seeped through the sandy clay, dissolving the minerals. Some of it got trapped in a roundish hole buried underground. This water had to drop its dissolved chemicals and leave them behind. Gradually these particles lined the rocky pocket with hard layers of tinted silica. Perhaps some of them arranged themselves in colored crystals of quartz.