Back in grade 4 science class, I had the joys of watching my teacher show us the Cartesian Devil. What is a Cartesian Devil? Well, it's a little toy to demonstrate physics, or impress your friends. At your command, you can control a little eyedropper in a bottle, with your brainwaves.

You will need three things: a clear plastic bottle, an eyedropper, and some soldering wire.

Fill the bottle up with water. Wrap the soldering wire around the eyedropper. Fill the eyedropper with enough water so that when it's in the bottle, the eyedropper barely floats (the rubber thing at the end of the eyedropper should barely touch the surface). Screw the top back on the bottle. You now have a fully functional Cartesian Devil.

Give the bottle a squeeze and watch the eyedropper sink to the bottom.

After a few years I had someone explain this trick to me. Apparently, air can be compressed, but water can not. The only air in the bottle is in the eyedropper, thus, when you squeeze the bottle, the air in the eyedropper compresses causing water to enter into the eyedropper. Does that make sense?