Florine Shelly, age 14, of Thoreau, N.M., for her question:

WHAT ARE EGGSHELLS MADE OF?

In nearly all mammals, the fertilized egg is small and stays inside the mother's body where the young mammal develops. Female birds and most insects, fishes and reptiles, however, force the fully formed egg out of the body.

A bird's egg is much larger than a mammal's egg because it contains food for the young bird to use while it develops outside the mother's body.

The shell is composed of two main layers. These layers contain pores so water and gases can pass through the shell. A thin film called bloom covers the pores, reducing the loss of water and gases. Eggs laid by different birds vary greatly in thickness of shell and the size and number of pores. When an egg reaches the uterus, a shell made of calcium and magnesium phosphates and carbonates covers the egg.