Cartesian diver experiment. The diver (here, a ketchup sachet counterweighted with paper-clips) sinks as the bottle is squeezed. This experiment is named after French philosopher and scientist Rene Descartes. Squeezing the sides of the bottle increases the pressure on the liquid inside, including the diver itself. This compresses the air bubble inside the sachet, causing the sachet to sink. Releasing the pressure on the bottle's sides causes the air bubble to expand and the diver will rise again. This experiment requires the sachet to have initially neutral buoyancy, hence the counterweighting with paper-clips.