I am surprised at how many people (chemistry faculty included) have never seen this demo. (oh, technically it is called a cartesian diver demo) Basically, you put some floating object that has an air space in a closed bottle of water. When you squeeze it, the diver goes down.
For my setup, I used a glass eye-dropper. Put it in a cup to make sure it just barely floats and then put it in a completely full water bottle. If you don’t have a eye-dropper, you can use anything that floats with an air space. I have done this with part of a straw before. Fold a small section of a straw in half so that air can’t get out of the top. Add a paper clip or something to the bottom so that it stays with the hole pointing down.
How does this work?
This could be explained on several different levels. I will go with the shortest.
When you squeeze the bottle, you increase the pressure in the liquid AND in the air in the diver. This makes the air bubble get smaller so that the diver displaces less water. The buoyancy force on the diver is equal to the weight of the water it displaces. Simple – no?