Why does your stomach puff out when you inhale?

Lungs expand during inhalation.

To make room for the lungs, the belly puffs out. Inside the rib cage are the heart and the lungs. Inside the abdominal cavity are our digestive organs. In between these two spaces is the diaphragm, the number one muscle for breathing.
 

The diaphragm is a unique muscle.

The diaphragm is dome shaped, sort of like a jellyfish.
When it is rounded up, in a classic dome shape, the diaphragm helps to push air out of the lungs during the exhale. To inhale, we pull the diaphragm downward, toward the belly. As the diaphragm flattens out air is sucked into the lungs. During inhalation the diaphragm flattens and pushes on the digestive organs in the belly. As the organs are squished by the diaphragm, the belly puffs out.

Nonstop Massage

Breathing actually massages the abdominal organs. How great is that?