The stomach digests pretty well everything sent down to it. How does it avoid digesting itself?

• THE STOMACH does not digest itself because it is lined with epithial cells, which produce mucus. This forms a barrier between the lining of the stomach and the contents. Enzymes, which make up part of the digestive juices are also secreted by the stomach wall, from glands with no mucus barrier. To prevent the cells which produce the enzymes from being digested in the process the enzymes are produced in an inactive form, as zymogens. It is only when they pass the mucus barrier lining the stomach, and reach the stomach proper, that the enzymes are activated by other secretions and they can begin digestion of the food in the stomach.

  Jo Munting, Norwich.

• They do, sometimes, when the mucus layer is broken- that's how ulcers happen (if my memory and A-level biology serve me correctly)

  Tom Chivers, Oxford UK