Cartesian Diver

PIRA 2B40.30

The Cartesian diver consists of a 2-liter pop bottle filled nearly to the top with water and an eye dropper filled with enough water such that it floats vertically. The amount of water in the eye dropper should reduce its boyancy enough such that the bottle doesn't need to be squeezed extremely hard to cause the 'diver' to sink.
When the bottle is squeezed, the 'diver' will sink. The volume of air in the top of the eye dropper will decrease as the pressure inside the bottle increases; hence, the boyancy of the eye dropper will decrease and it will sink.