The air inside a balloon wants to escape because of the difference in air pressure between the inside of the balloon and the outside of the balloon. When you blow up a balloon, you are forcing air molecules into the confined space inside, causing the air pressure inside the balloon to increase. The elastic material of the balloon stretches to accommodate the increased air pressure. The air pressure inside the balloon is higher than the air pressure outside the balloon, which creates a pressure gradient. The air molecules naturally try to move from an area of high pressure (inside the balloon) to an area of lower pressure (outside the balloon) to equalize the pressure difference. When you let go of the balloon opening and allow the air to escape, the high-pressure air inside the balloon rushes out through the opening, creating an action force. As the air escapes, it exerts a force on the air outside the balloon. According to Newton's Third Law, the escaping air's force has an equal and opposite reaction force. This reaction force acts on the balloon, propelling it in the opposite direction of the escaping air. The balloon moves forward as a result of this force, functioning like a rocket.