Walt Barefoot, age 11, of Florence, S.C., for his question:

HOW DO KIDNEY STONES FORM?

Kidney stones are deposits of calcium salts which occasionally form in the kidney or renal calculi. The cause of such deposits is unknown. It has been found that excessive amounts of calcium in the blood will not necessarily produce kidney stones.

The stones are irregular in shape with jagged edges. Most migrate downward and are found in the lower part of the ureter. Pain is sometimes associated with the blockage of the urine passage which often results from kidney stones.

Doctors say uric acid, cystine, fibrin and oxalate may also form kidney stones, in addition to calcium salts.

Doctors treat the kidney stone problem at first with drugs although surgery is sometimes necessary.