John Davies, age 11, of Lethbridge, Alberta, Can., for his question:

HOW DOES AN OYSTER MAKE A PEARL?

The oyster's body is soft and very sensitive, which is why the inside of his shell is lined with a pearly, nonscratchy surface. He constantly adds new layers of this pearly nacre to keep his walls satin smooth. Sometimes a grain of sand or some other scratchy little object gets lodged within his shells. The oyster solves this problem by surrounding the irritating foreign body with layers and layers of pearly nacre. After seven years or so, he builds it into a sizable pearl.