Cheryl Thompson, age 13, of Proctor, Minn., for her question:

WHERE DOES CHALK COME FROM?

Blackboard chalk that you use in school is a manufactured product made from such materials as gypsum and magnesia. It is used in such large quantities now that this is the best way to obtain the needed volume, but the original name comes from a product of nature.

Chalk is actually a fine grained, soft limestone. It is made of tiny shells of one celled animals although it often also contains shells of diatoms and parts of sponges. It is much softer and more porous than ordinary limestone.

Although it was often used in schools on blackboards, it is now used in glass, polish, tooth powder and paint. It is usually white, because of its marine fossil background, but can also be found in buff, flesh color or gray due to other minerals.