Cartesian Diver

Make an age-old toy: The Cartesian Diver. It floats in a bottle filled with water, but when you squeeze the bottle, the diver mysteriously plunges to the bottom.

Suitable for kids aged 5+ with parental supervision.

You Need:
  • An empty soft drink bottle with cap (any size, your diver will dive further in a bigger bottle)
  • A third of a straw
  • Paper clip
  • Small chunk of modelling clay
  • Glass or cup to test your diver in
  • Water
What to do:
  1. Fill the bottle with water.
  2. Make your Cartesian Diver by bending the third of a plastic straw in half. Holding the straw, bend the paper clip so its two U-shapes are separated. Slide one U-shape into each end of the straw to secure it. Mould a small amount of modelling clay or plasticine around the end of the paper clip. You want to completely seal the ends of the straws too, so no water can get into it.
  3. Fill the glass or cup with water and test your diver. This is the tricky step. You want the diver to barely float upright in the water (with only a small amount of the diver above the surface of the water). Add or remove plasticine to achieve this effect it could take a while! Note: a medicine bottle or plastic pipette will also work as your diver.
  4. Place the ready Cartesian Diver into the filled bottle of water and screw the lid on tightly. Your diver should be floating on the top, just like in the glass/cup you tested it in.
  5. Squeeze the bottle gently and watch as your diver plunges to the bottom of the bottle. Release your squeeze and he zips back to the surface. Can you control your diver so it stays at a particular depth? How about adding some food dye to your ocean bottle and perhaps decorating the outside in an ocean theme? Can you think of any other things you could use as your diver?

Why is it so?

In the beginning, the diver is buoyant in the water and floats on top because of the trapped air bubble making it light. As you squeeze the bottle and apply pressure, the air bubble in the straw becomes squashed and more dense. Dense air doesnt float as well as air that is more spread out because its heavier, and so the diver sinks. When you stop squeezing and remove the pressure on the air bubble, it spreads out, becomes less dense and the diver shoots back to the top.