1. Remove any labels from your bottle so that you can watch the action.
2. Fill the bottle to the very top with water.
3. Place a small pea-size piece of modeling clay at the end of the point on the pen cap. (see drawing)
5. Now screw on the bottle cap nice and tight.
6. Now for the fun part. You can make the pen cap rise and fall at your command. Squeeze the bottle hard - the pen cap sinks...stop squeezing and the pen cap rises. With a little practice, you can even get it to stop right in the middle.
Impressive, but how does it work? This eaxperiment is all about DENSITY. When you squeeze the bottle, the air bubble in the pen cap compresses (gets smaller) and that makes it more dense than the water around it. When this happens, the pen sinks. When you stop squeezing, the bubble gets bigger again, the water is forced out of the cap, and the pen cap rises.
The project above is a DEMONSTRATION. To make it a true experiment, you can try to answer these questions:
1. Does the size of the bottle affect how hard you have to squeeze the make the diver sink?
2. Does it matter if the bottle is not filled all the way with water?
3. Does the temperature of the water affect the density of the the diver?