1 liter plastic bottle
Plastic covered paper clips
2 sticks 8" long
See the Science Chapter for more instructions, hints, tips, and ideas.
The bottle needs to be straight especially near the bottom so it doesn't distort your view. If you can find a small bottle, it would make a neat neckerchief slide!
You may have to experiment with the amount of air to keep in the straw (Cartesian Diver). Too little air and it will sink to the bottom. Too much and it will be too difficult to make the diver sink.
When you squeeze on the bottle the diver should go deeper. If it is too difficult to get the diver to go to the bottom, then squeeze a little more air out of the straw.
As you squeeze on the bottle, you are increasing the internal pressure. Since water doesn't compress very easily, the air in the straw compresses. This reduces its volume and thus how much water it displaces. The less water it displaces the heaver it is and the deeper it will dive. You are balancing the floatation of the diver against the pull of gravity.
By squeezing on the bottle, you can control how deep the diver goes. Tilt the bottle to one side to catch the diver on the hook. To release the diver, simply turn the bottle upside down (the diver will float up from the bottom) then right-side-up.