Rene Descartes (1596-1650) was a French scientist and mathematician who used this same experiment show people about buoyancy. By squeezing the bottle, the test tube (diver) sinks and when released, the test tube surfaces. You can add hooks, rocks, and more to your set up to make this into a buoyancy game!

The test tube sinks because the when you squeeze the bottle, you increase the pressure of the water and this forces water up into the test tube, which then compresses the air inside the tube. When this happens, it adds enough mass to cause it to sink. Releasing the squeeze on the bottle means that you decrease the pressure and the water is forced back out of the tube.

Note: If you don’t have a test tube, try a medicine dropper! It’ll even work with a packet of ketchup (the kind you get from fast food restaurants), but since the air in the packet isn’t adjustable (it’s a sealed packet), get a couple so you can test them out.