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PySoy has a nice collection of environmental effects called "Fields" (in soy.fields). One of the nicer one is soy.fields.Buoyancy which has a small example at pysoy/examples/buoyant.py

One of the problems with this example is the field (aka, the liquid the objects are floating in) isn't visible. More mature examples such as Monopole.py the center is visible.

In this case the visual could be a completely opaque liquid (think "liquid paint") - code brevity is preferred over nice visuals, but something which clearly shows what parts of the objects are above the liquid and which are not.

When you're done, attach the "hg bundle" of your work to this task.