How do you breathe? Take a deep breath. Feel the air rushing in? How does it feel? Cool? Crisp? Now blow out. Do you feel the warm air blowing over your lips? As you breathe in, a special organ called a diaphragm pulls your lungs down, making them bigger. This causes air to rush into your lungs. Think about this for a moment.
Imagine you are taking a bath. Sitting on the side of the tub is an empty shampoo bottle. What happens if you squeeze the shampoo bottle and then stick it under water? Nothing, right? But now suppose that you let go of the sides of the bottle while keeping it under water. As the sides of the bottle expand, the shampoo bottle fills with water.
As your lungs expand, they fill with air. If you were to again squeeze the bottle what would happen? The water would squirt out, wouldn't it. When your diaphragm again squeezes the bottom of your lungs, air is forced out of them.