Try this activity from the Exploratorium to see a surprising result. You'll make a Cartesian diver, a tube sealed in one end that barely floats when placed in water.  The diver, made from an eyedropper or Bic™ pen, will float, sink, or hover in a soda bottle in response to pressure changes.  The site also provides an explanation of what happens, suggestions for extensions, and a connection to the buoyancy of ships. This activity is part of Exploratorium's Science Snacks series.