What you’ll need

  • A balloon
  • A straw
  • A piece of string 3 to 5 meters long that passes easily through a drinking straw, e.g., linen thread or butcher’s twine
  • Adhesive tape (e.g., masking tape)
  • Scissors

How to do it

1. Attach one end of the string to a stable object, e.g. a door handle, window handle or chair back. Make sure that you can stretch the string across the room, because you’ll need plenty of space to carry out the experiment.

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2. Thread the loose end of the string through the straw.

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3. Blow up the balloon and pinch the end with your fingers to prevent any air from escaping.

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4. Fix the straw to the balloon using some adhesive tape. The loose end of the string must point towards the opening of the balloon.

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5. Slide the balloon along the string towards the loose end of the string. Tighten the string and remove your finger from the opening of the balloon. What happens?

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Well spotted!

When you let go of the balloon, it zips along the string as air escapes.

What's the secret?

When you inflated the balloon, you'll have noticed how strongly you had to blow to get the air into it. And the more air you blew into the balloon, the more it pushed against the sides from inside. When you let go of the opening, the air quickly escaped from the balloon as a jet of air. This jet of air functions like jet force and propels the balloon forwards.

Rocket engines function according to the exact same principle: as the fuel burns inside the rocket, the gases produced escape through a nozzle and the rocket is propelled in the opposite direction. An octopus moves according to the same principle: by absorbing water into its body and then expelling it suddenly.