An oversimplification to describe jet propulsion is to imagine an inflated balloon when the air is suddenly discharged. The air pressure in a closed balloon is equal in all directions, but when the stem is released the internal pressure is less at the open end than at the closed end, causing the balloon to shoot forward. The difference between high and low pressures inside the balloon is what propels it. Newton's third law which states that every force must have an equal and opposite reaction, shows that this rearward force is balanced by a forward reaction known as thrust. The power source for a jet engine is a high energy fuel, burned at intense pressures to produce large gas volume.