What is the acid made by the stomach to let pepsin work?

The acid produced by the stomach to enable pepsin to function is hydrochloric acid (HCl). This strong acid creates an acidic environment with a pH around 1.5 to 3.5, which is essential for activating pepsinogen into pepsin. Pepsin, in turn, is an enzyme that helps break down proteins into smaller peptides, facilitating digestion. The acidic environment also aids in killing harmful bacteria ingested with food.