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Ball in the Balloon

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.


  • Describe the characteristics of air.

  • Explain how air pressure works.

  • Discuss how air pressure affects our daily lives.


  • Per Demo or Group:
    dark coloured balloon (opaque)
    bouncy rubber ball

Key Questions

  • What could be keeping the balloon from deflating?
  • Now that you’ve figured out that something is blocking the air from coming out, what should I do to let the air come out?
  • Does turning the balloon upside down support your theory?
  • What is keeping the ball in the opening?
  • In which direction is the air (in the balloon) pushing?
  • How do I let the air out?

What To Do


  1. Squeeze a rubber bouncy ball into the dark coloured balloon. Do not let the students see you do this.


  1. Blow up the balloon. Do not tie it off.
  2. Hold the balloon upright so the ball descends and covers the opening. The balloon should stay inflated. Ask the students to speculate about how this is happening (think, pair, share). They may guess that something is acting like a stopper.
  3. Turn the balloon upside down, with the neck up. The air pressure in the balloon will hold the ball in place, keeping the balloon from deflating. Ask the class to explain what is happening.
  4. Tap the balloon so the ball falls down and the air comes out. Show the students the outline of the ball in the balloon.


  • When you let go of an inflated balloon WITHOUT a ball inside, the elasticity of the balloon forces the air out and pushes the balloon forward. Use this principle to make balloon-powered rockets and boats.