A Cartesian diver or Cartesian devil is a classic science experiment, named for René Descartes, in which the "diver" - a small tube, open at one end such as an eye dropper - is placed in a much larger container with flexible walls, such as a 2-liter soft drink bottle, and adjusted so it is neutrally buoyant, or is neither floating nor sinking at the top 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 latex rubber or some other elastic or flexible material). The device also has a practical use for measuring the pressure of a liquid. In addition, the principle is used to make small toys often called water dancers or water devils. If the tail of the glass bubble is given a twist, the flow of the water into and out of the glass bubble creates spin. This causes the toy to spin as it sinks and rises. An example of such a toy is the red "devil" shown to the left. It is topologically identical to the eye dropper.
How the Cartesian diver works
The diver is an object in a sealed container of water. Air in the diver makes it buoyant enough to barely float at the water's surface. When the container is squeezed, the pressure compresses the air and reduces its volume. This permits more water to enter the diver, resulting in it being less buoyant and sinking. This, being known as density, increases how much air would be in 1 square centimeter of space in the diver, also increasing the weight of the diver, overruling the buoyant force on it and making the diver sink.
