Stomach

The stomach is a muscular, hollow, dilated part of the digestion system which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects (mid-gut), and molluscs. It is involved in the second phase of digestion, following mastication (chewing).
The stomach is located between the esophagus and the small intestine. It secretes protein-digesting enzymes and strong acids to aid in food digestion, (sent to it via oesophageal peristalsis) through smooth muscular contortions (called segmentation) before sending partially digested food (chyme) to the small intestines.

Flatulence

Flatulence is the state of having excessive stomach or intestinal gas. This can result in uncomfortable feelings of bloating, as well as increased belching (burping) or passing of gas from the rectum. Flatulence occurs when a food does not break down completely in the stomach and small intestine. As a result, the food makes it into the large intestine in an undigested state. For example, if you are "lactose intolerant," it means that you lack an enzyme (lactase) in your intestine -- the enzyme that breaks lactose apart into two sugar molecules so they can enter the bloodstream. Without lactase, lactose passes undigested through the stomach and small intestine and arrives in the large intestine.