You will need:
- A small soft drink bottle
- A Plastic pipette
- Water
- Scissors
- Two nuts to fit the pipette.
- You can use a pen lid with plasticine to weight it down, but it is not as effective.
Cut the length of the pipette off, leaving 2cm before the bulb.
Fit two metal nuts to the bottom of the cut pipette (or add plasticine to the pen cap).
Fill the bottle with water all the way to the brim.
Float the cut pipette in the water and adjust the weight to allow it to float.
Close the bottle and tighten the lid. Ensure that there is no air gap below the lid at all (otherwise you’ll be compressing that air as well and it be harder for the activity to work).
Squeeze the bottle and the cartesian diver should sink, letting go the cartesian diver should rise.
As you squeeze the bottle, look inside the pipette and you should see the air bubble get smaller inside. Once you stop squeezing the bottle, the air bubble expands and the cartesian diver becomes more buoyant.
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Why Does This Happen?
This toy is also known as a SCUBA Sally, Diver Dan or simply a Cartesian Diver.
The toy works on buoyancy. Squeezing the bottle exerts pressure throughout the contained fluid. This pressurized fluid rises into the opening at the bottom, 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 its buoyancy and allowing it to float.
We’ve seen this same experiment done with tomato sauce packets, biro lids and 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!
Variable testing
- Try large vs. small bottles. Which is easier?
- What happens if you add more weight to the pipette or pen lid?
- Can you use another device with an air gap inside?
- Does this still work if you change to another liquid?
- Teaching about air pressure? Check out the Flight or Weather show!
- Teaching about Newton’s laws? Check out the Forces, Friction & Movement workshop!
Get in touch with FizzicsEd to find out how we can work with your class.
Learn more about air pressure!
Get the 60-minute video + PDFs + curriculum links for your class here!
Working with Water
Years K to 2
Maximum 30 students
School workshop (NSW & VIC)
60 or 90 minutes
Online Class Available
STEM Full Day Accelerator - Primary
Designed from real classroom experiences, this modular day helps you create consistently effective science learning that directly address the new curriculum with easily accessible and cost-effective materials.
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