Have you ever wondered how you breathe? Yes, air goes into your lungs, but what causes the air to go into your lungs? The answer is quite simple. Your lungs are in an air tight container formed on the sides by your ribs and at the bottom by your diaphragm. When you breathe, you cause the container to expand — you pull your diaphragm down and the ribs expand outward. That causes the air pressure in the container to decrease. Nature abhors a vacuum. Air naturally flows into the lungs to stabilize the air pressure.
The next question, then, is how do you expand the container. To understand that, I highly recommend this video: