Illustration of stomach, pyloric valve and upper part of small intestine (duodenum)

Your stomach is a muscular sac about the size of a small melon that expands when you eat or drink to hold as much as a gallon of food or liquid. Once your stomach pulverizes the food, strong muscular contractions (peristaltic waves) push the food toward the pyloric valve, which leads to the upper portion of your small intestine (duodenum).