I remember having a Cartesian diver toy when I was a kid. Now, you can make your own. This Cartesian diver craft comes from a Japanese site and has photos and instructions in English and Japanese. One advantage of living in Japan is when you buy sushi at a supermarket, it always includes a little soy sauce packet. These are often little plastic tubes shaped like fish. They have a resealable screw on cap and are perfect for making the little Cartesian divers. You can also buy them at the store. They need a little weight at the bottom like a nut and need enough air inside to just barely float to the surface. Put them in a sealed plastic bottle full of water. When you squeeze the bottle, they sink. You can also get fancy with some divers with little hooks and try to snag weights on the bottom like a game. Visit this site today for the full tutorial, have fun.