To see the effect of pressure on air density.
This very old and well known experiment, `Cartesian Diver' gets its name after the French Philosopher, Rene Descartes (1596–1650).
flexible bottle, straw, paper clip
Fold a piece (6 cm) of straw in half and join the inner walls of the open ends with a paper clip (the ends should remain open). This is your diver. You may have to adjust the weight of diver for it to work. Put it in the bottle filled three-fourth with water. Adjust the quantity of water in the diver (by squeezing), so that tip of the diver stays above water surface. Cap the bottle. Squeeze the bottle, the diver sinks; release it, diver comes up.
When you squeeze, pressure inside the bottle increases and some water enters the diver forcing air inside it to shrink. Divers density increases, so it sinks.
When you release, air trapped in the diver expands, pushing out the extra water. Diver's density decreases and it rises up.