How does a Cartesian diver work?

According to William Orem for Indiana Public Media, the Cartesian diver works by demonstrating the differences between density measurements in air and water. The experiment exerts and releases water pressure onto remaining air particles trapped in an apparatus such as a straw or eye dropper that has been placed inside a water bottle, causing the apparatus to rise and sink in response to the applied pressure changes.

The Institute of Physics further explains that water is much denser than air. The Cartesian diver is a simple experiment designed to illustrate this effect. When pressure is applied from outside of the water bottle through squeezing motions, a density change is created by increasing the pressure throughout the water contained within the bottle. This pressure change causes the water to push against the air within the selected flotation mechanism, which forces the apparatus to rise upward under the force of the water. When the pressure is released, the density declines to its original state, and the flotation device sinks back down into the bottle. The experiment is common in middle and high school physics courses and is often recommended for children to try at home. It was named after René Descartes, who was a 17th-century French philosopher, mathematician, scientist and writer.