Cartesian diver
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A Cartesian diver is a classic science experiment, named for René Descartes, in which an eye dropper or other container open only at the bottom (the "diver") is placed in a much larger container with flexible walls, such as a 2-liter soft drink bottle, and adjusted so it barely floats at the top of the water in the larger container.
When the larger container is squeezed, the air inside the diver is compressed, reducing the overall displacement or buoyancy of the diver, which then sinks. One reason for using an eye dropper is that air cannot readily escape the end of the dropper, due to surface tension, however a pen cap can also be used.
The device also has a practical use for measuring the pressure of a liquid.
A 2004 toy called Aquapets is based on the principle of the Cartesian Diver.de:Cartesischer Taucher