Cartesian diver
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A Cartesian diver or Cartesian devil 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.
Note that this experiment can also be done with a rigid bottle and a flexible cap (made of cork or some other elastic or flexible material).
The device also has a practical use for measuring the pressure of a liquid.
[edit] How the Cartesian diver works
The diver is an object suspended in a chamber of water, which is buoyant enough to barely float at the top of the chamber. When the chamber is squeezed, the pressure on the air captured in the diver increases, which eventually makes the diver neutrally or negatively buoyant. As the air in the diver becomes pressurized its volume is decreased, causing the diver to sink.
