Julie Canfield, age 11, of Staten Island$ N.Y., for her question:

WHAT CAUSES CANKER SORES?

Many people develop shallow ulcers on the tongue, lips or elsewhere in the mouth. These spots, called canker sores, start as blisters and then become raw, reddened and painful.

We don't know what causes canker sores, and why some people get them and others do not. We know a similar type of common eruption of the mouth, called a fever blister, is caused by a virus.

Canker sores tend to recur from time to time, disappearing after a few days or several weeks. Some people control them by eliminating such things as chocolate and nuts from their diets. With others, diet has no effect.