Tracy Leslie, age 14, of Duluth, Minn., for her question:

HOW DOES A BONE HEAL?

Tracy's brother recently broke a bone, the doctor set it, and it's now on the way toward being healed. Within days or even hours of an injury, a meshwork begins to form from the clotted blood, lymph and other substances in the area of the fracture. This meshwork begins to stabilize the bone ends, and calcium is soon deposited to form what is called a callus.

 No one knows exactly how this wonderful healing process works, but calcium continues to be deposited and become thicker  and soon the break is closed. General health, old age or other illness does not affect the healing process although children heal much more quickly than do adults.