When a paper airplane is flying, there's a delicate balance between the downward force of gravity on the plane and the upward force of the air.

A great insight from physics is that all of the forces of gravity acting on every atom of the plane behave like one force pulling down at a point called the "center of gravity."

This point is easy to find. The plane will balance on one finger at its center of gravity (or on two fingers straddling its center of gravity).

Balance the plane on your finger or fingers like the diagram. Mark this balance point with a pencil.