Now that you have a flying plane, you can use it to see the roles that paper plays in its construction. After the kinetic energy (that is, the energy you put into it by throwing it) of the initial throw has dissipated, paper planes are gliders powered by gravity. As the plane falls, its wings deflect air backward and down, providing thrust and lift. Paper makes a good wing because it's impermeable to air: In a single sheet of paper, multiple layers of interlocked fibers prevent air from flowing through. In contrast, a hole-filled screen from a back door would not make a very good wing.