Robin Kincaid, age 11, of Allentown, Pa., for her question:

WHAT MAKES YOUR STOMACH GROWL?

The stomach muscles churn the food and digestive juices into a pulpy liquid. Then the muscles squeeze the liquid toward the intestinal end of the stomach by ringlike contractions of the muscles. These contractions, called peristaltic waves, occur about 20 seconds apart. They start at the top of the stomach and move downward. The pylorus, a ringlike muscle around the duodenal opening, keeps food in the stomach until it is a liquid. Then the pylorus relaxes and lets some of the liquid digested food, called chyme, pass into the duodenum.

The churning action tends to begin at our usual mealtimes. When a person says his stomach is ''growling,'' he is referring to these peristaltic waves. Sometimes these movements grow so strong that they squeeze acid gastric juices up into the lower part of the esophagus.