Last time we explained how to make a Cartesian Diver. You fill a clear plastic bottle with water and place an eye dropper in it. The eye-dropper should have just enough water inside it so that it barely floats.

Now seal the bottle tightly and squeeze on its sides. The eye-dropper sinks! Stop squeezing and the dropper rises once more. Today we'll explain why this works.

How Does It Work?

The Cartesian diver, named after French philosopher and scientist René Descartes, works because of several factors.

When you squeeze the sides of the bottle, you are increasing the pressure on the liquid inside. That increase in pressure is transmitted to every part of the liquid. That means you are also increasing pressure on the eye dropper itself.

Squeeze hard enough and you will push some more water up inside the dropper. The air inside the dropper squeezes tighter as more water is forced in.

Increasing The Density

Now, water is much denser than air. So when you push more water inside the dropper, you increase its overall density. Once its density is greater than that of its surroundings, it will sink.

Release the pressure on the bottle's sides and you stop forcing water inside the eye- dropper. The air inside it will now push out the extra water again, and the eye-dropper will rise. That's the Cartesian Diver!