Even though you can't see it, air is not just empty space.
Air is a fluid, like water.
Although its invisible, air is made of tiny particle and molecules that have mass and weight. Mass is the amount of stuff something is made of. Like water, air molecules flow, take different shapes and are affected by outside pressure. The same balance of forces that affects a boat affects an airplane. Air particles and molecules constantly bounce around and bump into each other, exerting a large amount of pressure and force.