Today we did a fun experiment which demonstrates how applying external pressure to a container it increases the density of the object within it causing it to sink.
First we filled an empty plastic pop bottle (small) with water almost to the top. We dropped paper clips into the bottle. Next we taped a small magnet to a Styrofoam packing 'peanut' and screwed the lid back on the bottle.
We wanted to make the 'diver' go down to the bottom and retrieve the paperclips. How would we do this? By squeezing the bottle and holding it squeezed this increases the pressure and forces the diver down to the bottom. When you release the bottle the diver returns to the surface bringing with it paperclips attached to the magnet.
Here G demonstrates quite theatrically for us!