Your stomach gets a brand new lining every few days

Your stomach would digest itself if it did not rebuild itself constantly. The strong hydrochloric acid that breaks down food would eat through tissue, so your stomach lining is replaced every 3 to 4 days. Special goblet cells produce a thick layer of mucus-bicarbonate that shields the wall, and epithelial cells divide rapidly underneath. When that mucus barrier fails, you get an ulcer. In fact, the classic reason ulcers were misunderstood for years was because we missed that a bacterium, H. pylori, can burrow through that protection.