Air occupies a certain space, but it has no fixed shape and volume. When pressure acts on the air in the closed container, the volume of the air is compressed and the internal pressure increases. When the external force is removed, the air will return to its original volume under the action of internal pressure. If there is a movable object in the container, when the air returns to its original volume, the object will be pushed out and ejected by the pressure of the air in the container. This principle is widely used in production and life. For example, injecting compressed air into the ball, the more air, the harder the ball; When compressed air is filled into the tire, the tire can bear a certain weight. For large cars, use compressed air to open and close the doors and brakes; The hydraulic press uses compressed air to pressurize the water. In the factory, compressed air is used to start the air hammer to strike iron. In the coal mine, it can start the pneumatic pick to drill holes. Compressed air is also used for pipeline transportation of liquid and granular objects.

My teacher approved it, and it was right.