Stomach acid is strong enough to dissolve metal, yet it doesn't dissolve your stomach.

The stomach produces hydrochloric acid at a pH as low as 1 to 2, acidic enough to corrode a razor blade or dissolve a piece of zinc over time, and strong enough to break down tough food proteins and kill most ingested bacteria. It’s one of the most corrosive substances the human body makes.

The stomach protects itself with a thick layer of mucus, roughly a millimeter or two deep, that coats the stomach lining and keeps the acid from touching living tissue directly. Cells in that lining are also replaced constantly, with the surface layer typically renewing itself every few days, so minor damage is repaired before it becomes serious. When that mucus barrier is weakened — commonly by bacterial infection or long-term use of certain pain medications — the acid can begin to erode the stomach wall, leading to ulcers.