Take a 2 liter bottle and fill it with water up to 2/3 level. In the kit, there is a small plastic bottle, with a cap. This is a
'Cartesian diver'. In 2 liter plastic bottle filled with water, gently drop this Cartesian diver upside down. Cap the plastic bottle tightly. You will find that the Cartesian diver will sink a little bit and remain just below upper level of the water. Press the bottle gently and notice a miracle. The Cartesian diver dives deep into the water and touches bottom of the bottle. As you release the bottle, it bounces back to its original position. You can repeat this fun exercise till you understand science behind this miracle.
How does it happen?
The Cartesian diver has a cap with a central tube. Initially, the diver is in the top layer of water. The diver is filled with air. We know that air also has some weight. Being it is lighter than water, the diver rests in the top layer. When we press the bottle, air in the plastic bottle is compressed. It exerts pressure on water in the bottle. Water enters the diver making the diver heavier. So the diver sinks. As you release pressure from the plastic bottle, the water from diver comes out and it becomes lighter. So it bounces back to its original position. It is a classic experiment in properties of air. It is named after the scientist, Rene De-cartes (1596-1650). We learn that air has weight, it exerts pressure and it can be compressed. This diver is also useful in understanding concept of buoyancy and even Archimedes principle.