Jacqueline Casto, age 12, of Wichita, Kansas,, for her questions

What causes a pearl to form?

Pearls are made by oysters and other shell fish. All these little pearl makers live in hard shells which often look very rough and dirty on the outside. The inside of the shell houses however, is another matter. The walls are lined with hard glossy material called nacre. This is necessary, because the body of the little pearl maker 3s very soft and sensitive. Rough walls would scratch him   which is why he is always repainting his inside walls with hardy glossy nacre,

The pearl maker extracts the materials for making his nacre out of the water. It is processed and secreted through special cells. If a grain of sandy a little worm or other scratchy object gets inside the shells it too gets a coat of nacre. It gets a new coat every time the house is painted. Gradually it builds up, layer upon layer of beautiful nacre until it becomes a round lustrous pearl.