The Cartesian diver demonstrates not only buoyancy, but the implications of the ideal gas law and Pascal's principle as well. Squeezing on the top of the sealed plastic container decreases the volume and therefore increases air pressure above the water. By Pascal's principle, that pressure is transmitted to all parts of the container. This increases the pressure inside the small glass vial. The increased pressure decreases the volume of air at the top of the vial, and in so doing, decreases the amount of water displaced by the vial. This decreases the buoyant force on it enough to cause it to sink.