Jeff Sargent, age 13, of Lynn, Mass., for his question:

WHERE DO WE FIND SILICA?

Silica is the name for silicon dioxide. It is the most abundant and widely distributed substance in the earth's crust. We find that 60 percent of the earth's crust is silica. We actually find silica almost every place we look.

Silica is the material in the formation of most common and crystalline rocks. Crystalline silica forms the minerals coesite, cristobalite, quartz, tridymite and others.

As quartz, silica make up the largest part of sand. Pure quartz sand is used in making glass and firebrick.

Crystals of quartz, both natural and artificial, are used to keep radio transmitters and receivers tuned.

Silica combines with other substances to form minerals called silicates. It gives strength to some plant stalks, the quills of feathers and some living sponges. It also makes up the shells of some microscopic forms of life.