What you need:
A one-litre plastic bottle, An eyedropper, A vessel, Water
What to do:
1. Fill the bottle with water.
2. Fill some water in the vessel too. Fill water in the dropper and drop it in the vessel. Adjust the water in the dropper so it floats in the water.
3. Now, put the dropper into the bottle. Fill the bottle to the brim with water and close its cap.
4. Place the bottle upright on a table or a flat surface and wrap one hand around it. Gently, squeeze the sides of the bottle.
What happens:
The dropper dives to the base of the bottle when you squeeze it and floats back up when the bottle is released.
Why?
When you squeeze the bottle, you apply pressure on it. This causes the water in the bottle to get forced into the eyedropper (you can see the water level inside rise if you watch carefully). The air in the eyedropper also gets compressed and pushed to the top of the dropper because now more water is sharing its space. This increases the mass of the dropper and its density (i.e. the number of molecules packed into a small space). And dense objects sink. When you release the squeeze, the pressure on the dropper is released too. The extra water flows out of the dropper and the air is able to decompress. This lowers the density of the dropper and it can float once more.
In this case, the eyedropper acts as a ‘Cartesian Diver. Named after Rene Descartes, a French philosopher and mathematician, a Cartesian diver is an object that demonstrates the laws of buoyancy in a liquid.
Picture Credit : Google
