Equipment typical required (per demonstration):
The object needs to be placed inside the bottle and and the bottle then filled completely with water and capped. It is essential that there is no air left in the bottle otherwise when squeezed, this air will also compress.
When the sides of the bottle are squeezed, the pressure is applied to the water but because water cannot be compressed, this compression is transferred to the air inside the diver. This air compresses and the object becomes denser until it reaches a point where it cannot float. It should sink some way down the bottle.
Some trial and error may be required because ketchup sachets vary greatly from one to the next, some containing more air, some less. If you are making your own diver, plasticine may be added to get the correct buoyancy to begin with. Students may want to experiment with different shaped divers or different materials which to make one with.
This demonstration may be conducted to show how air can be compressed while water cannot and also as an introduction to Pascal's Principle and hydraulics.