Why doesn't your stomach digest itself?

The acid in your stomach is strong enough to burn your skin, yet it lives inside you all day without eating through the wall.

Your stomach pumps out hydrochloric acid at pH 1 to 3 to break down food. The wall survives because surface cells coat it with a slippery layer of mucus and bicarbonate that neutralizes the acid right at the lining, keeping that surface near neutral. The barrier wears down and gets rebuilt constantly, and when it fails, acid attacks the wall and you get an ulcer.

Squeeze lemon juice (pH about 2) on a cut and it stings - your stomach acid is that strong or stronger, but the mucus coat keeps it off the wall like grease on a pan.

Stomach acid is genuinely corrosive; a renewable mucus-and-bicarbonate layer, not tough tissue, is what protects the stomach.

It explains why ulcers happen and why painkillers like aspirin or ibuprofen, which weaken that protective layer, can let the acid bite into the stomach wall.

Acid in the tank, slime on the walls - no slime, ulcer.

Learn the idea and practice English at the same time.