10 minute STEM: Cartesian Diver

Cartesian Diver

Here is a very quick and easy science experiment you can do at home with some basic household supplies.


  • plastic water or soda bottle with twist off cap
  • large deep measuring cup or bowl
  • sauce packets OR eye dropper
  • water


  1. Collect a bunch of sauce packets, ketchup, soy sauce etc. Must be in a standard plastic packet found at most fast food places.
  2. Fill a large measuring cup up with water, enough that the packets can float freely.
  3. Test your packets in the measuring cup. Not all packets are filled evenly so you need to find a packet that just barely floats. It should be sitting vertically with just an edge sticking out.
    1. If you are using an eyedropper fill the dropper so that it is also barely floating.
  4. Add water to the water bottle, fill most of the way up. Place  the sauce packet gently in the bottle. The sauce packet should still float.
  5. Fill the water bottle the rest of the way up. The bottle MUST be filled to the brim.

    Fill the bottle to over the brim. Careful, some will spill out when you close it!

  6. Tighten the cap on the bottle.
  7. Challenge family members to make the sauce packet sink to the bottom of the bottle. Most won’t be able to figure it out!
  8. Show off the secret trick…… Squeeze the sides of the bottle!Note: I’m using the bottle upside down because the ridges in the side of the bottle kept catching the packet when right side up. 

How does it work?

The Cartesian Diver is named after French scientist and philosopher René Descartes. Cartesian Diver’s are an example of Boyle’s Law. Basically Boyle’s Law says that the volume and pressure of a gas (like air) is inverse. Meaning that when you increase one, the other decreases. Increase volume, pressure decreases or increase pressure, volume decreases.

Okay, but how does it work?

Because you filled the bottle all the way up, there is no room for the water to move. When you squeeze the bottle, the pressure builds up and water squeezes the packet. The air in the packets is made smaller and the packet becomes denser. The packet then becomes too dense to float, so it sinks!


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