What is the reason for the outward rise and inward fall of people's belly alternatively with inspiration and expiration?
The outward rise and inward fall of the belly during breathing is due to the movement of the diaphragm muscle. When you inhale, the diaphragm contracts and moves downward, pushing the abdominal organs outward. When you exhale, the diaphragm relaxes and moves upward, allowing the abdominal organs to move inward.