The stomach is a small hollow bag with muscular walls. As you eat it stretches to hold up to 3 litres. It is both a food mixer and acid bath. Chewed food is churned and mixed with powerful enzymes and digestive fluids to become a thick liquid called chyme. Gastric acid is strong enough to destroy the stomach itself, which is why protective mucus lines the stomach. After several hours, the chyme passes from the stomach into the small intestine.