Melissa Simpson, age 13, of Twin Falls, Ida., for her question:

WHAT CAUSES RICKETS?

Rickets is a bone disease that occurs mostly in children under 3 years of age. Often it is caused by a lack of calcium and vitamin D or by the inability of the body to use these substances properly.

The bones are so soft that they bend into abnormal shapes. Rickets results in conditions called bowlegs, knocknees, chicken breast, funnel chest, rosary ribs and knobbed forehead. As the child grows, ' bones harden but the abnormal shape usually remains.

Doctors use X rays to diagnose rickets.

Eating foods that are rich in calcium and vitamin D usually prevent rickets. Calcium can be obtained by drinking plenty of milk and eating green vegetables. The best sources of vitamin D are sunlight and fish oils.