Randy Knight, age 10, of Charlotte, North Carolina, for his question:

Where does chalk come from?

It was made by tiny creatures who lived in the sea, millions of years ago. They are classified as foraminifera, a name that refers to the tiny holes in their tiny shells. These creatures absorbed calcium from the water and used it to build shells of calcium carbonate around their soft bodies. They kept in touch with their watery world by poking fingers through their windows.

About 100 million years ago, the seas flooded much of Europe and about half of North America. Zillions of foraminifera thrived in these shallow waters and thick layers of their chalky white shells sank to the bottom. When the seas returned to their basins, these deposits became dry land    and formed thick layers of chalk. This was the Cretaceous, or chalk making period of the earth's history. Most of our chalk beds were made at that time, though countless foraminifera still carry on their work in our salty seas.