Cartesian Diver


2L pop bottle with cap
eye dropper


1) Fill the pop bottle all the water. Insert the eye dropper, making sure the dropper is about half-full of water. Screw the cap on the bottle tightly.
2) By squeezing the sides of the bottle, you can make the eyedropper plunge to the bottom, float back to the top, or even hover halfway!
3) You can make this look like a magic trick by holding the bottle in one hand and a pen or other object in the other hand. Tell your audience that you can make the dropper move by magic. Then point the pen at the dropper while at the same time squeezing the bottle. As the dropper descends through the bottle, move the pen accordingly. If you do this carefully, you can make it look like the pen is moving the dropper, not you!

Explanation: Liquids, such as water, are incompressible - meaning their volume is constant. Gases, like air, are compressible. So when you squeeze the closed bottle, the water can't be squished any smaller - but the air inside the eyedropper can. This makes more water flow into the bottom of the dropper, making it heavier, and sending it to the bottom of the bottle. As soon as you release the pressure on the bottle, the dropper floats back to the top!

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