Melissa Buckman, age 13, of Erie, Pa., for her question:

WHERE DO WE GET CHALK?

Chalk is a soft, white type of limestone that has been formed on the bottom of ancient oceans. It is made of the shells of tiny one celled animals known as Foraminifers. These creatures have shells that are made of calcium carbonate. They die, sink to the bottom of the sea and are mixed with lime mud to form chalk.

Blackboard chalk may be true chalk, but often it is made from plaster of Paris. Plaster of Paris is made from a mineral called gypsum.

The world's best chalk deposits were formed during the Cretaceous geologic period that started 135 million years ago and lasted 65 million years. The dinosaurs lived during this period. Cretaceous comes from the word "creta," the Latin word for chalk.

The world's most famous chalk deposits are the white cliffs of Dover, England. Across the channel on the French coast are other great chalk cliffs.

Chalk is used in making some paper, rubber goods, paint, putty and linoleum. It is also used as a soft polishing powder and in tooth powder.