In this demo, students must brainstorm an explanation for the mysterious behaviour of a balloon that stays inflated without being tied.
The answer lies in the force that is exerted by air inside the balloon.
When a rubber bouncy ball is placed inside of a balloon, the ball falls toward the opening, acting as a stopper by preventing the air from escaping even if the balloon is not tied.
Since the air pressure inside the balloon is greater than its surroundings, it pushes against the inner walls of the balloon trying to equalize the low-pressure air around it. The ball is basically "in the way" and is pushed against the opening by the air inside the balloon. In other words, the air molecules in the balloon exert a force on the ball, keeping it in place.