Dianna Booker, age 13, of Providence, R.I., for her question:

WHAT CAUSES NAUSEA?

Nausea is perhaps best described as that disagreeable sensation of feeling sick to the stomach. There can be many causes of nausea, both physical and mental.

Mental causes include revolting sights, disgusting odors and sudden fright. Physical causes include severe pain arising anywhere in the body, blockage or obstruction of the digestive tract and the stimulation of our balancing organs, the semicircular canals of the ears.

In nausea, the stomach muscle contracts gently in a direction that is opposite of its normal contractions. A stronger contraction in this direction can cause vomiting.

The sensation of nausea may be produced by stimulating certain nerves which have their centers in the part of the brain called the medulla. It may come from the digestive tract or other parts of the body, from higher centers of the brain or as a direct result of outside agents such as the drugs morphine and digitalis.

The tendency to nausea varies widely among different persons. Some can endure the tossing of, a ship on the roughest sea without discomfort. Others are nauseated merely by the gentle swaying motion of a swing or hammock.