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Make a cartesian diver

You will need:

  • A small soft drink bottle, a plastic pipette, water, scissors and 2 nuts to fit the pipette
  • You can use a pen lid with plasticine to weigh it down but it is not as effective.



  1. Fill the bottle of water all the way to the brim.
  2. Cut the pipette as shown in the video.
  3. Fit 2 metal nuts to the bottom of the pipette (or add plasticine to the pen cap).
  4. Float the cut pipette in the water... adjust the weight to allow it to float.

    diver dan experiment closeup
    Perfect height for the pipette in the bottle
  5. Tie the bottle cap onto the bottle.
  6. Squeeze the bottle and the 'diver dan' should sink, letting go should allow it to rise.

    diver dan diving activity
    A little squeeze and the pipette diver sinks!
Be Amazing Book Front Cover; Ben Newsome, teacher & founder of Fizzics Education. Be Amazing -how to teach science the way primary kids love
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How to teach science, the way primary kids love

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Why does this work?

This toy is also known as a SCUBA Steve or simply a Cartesian Diver.
The diver dan is based on buoyancy. Squeezing the bottle exerts as pressure throughout the contained fluid. This pressurized fluid rises into the opening at the bottom of the , making the pipette heavy and sinking it. Releasing the bottle reduces the pressure around the pipette, allowing the trapped air inside the pipette to expand, increasing it's buoyancy and allowing it to float.

We've seen this same experiment done with tomato sauce packets, biro lids, Freddo frog packets... really any small trapped air space can work, you just need to be able to fit it into the bottle and weigh it down to keep it floating upright. Have fun!
Other air pressure based science experiments
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