The scientific principle on which the jet engine operates was first stated in scientific terms by Sir Isaac Newton in 1687. According to Newton's third law, for every action, there is an equal and opposite reaction. That principle can be illustrated by the behavior of a balloon filled with air. As long as the neck of the balloon is tied, gases (air) within the balloon push against all sides of the balloon equally, and no motion occurs. If the neck of the balloon is untied, however, gases begin to escape from the balloon. The escape of gases from the balloon is, in Newton's terms, an "action." The equal and opposite reaction resulting from the escape of gases is the movement of the balloon in a direction opposite to that of the movement of the gases. That is, as the air moves outward in one direction, the balloon moves outward in the opposite direction.