Thanks, all, and Mark, your solution is brilliant, as always!  I'd thought about moving the hotpoint to the edge, but as Mark says, the balls roll while they're moving, so hotpoint to the "bottom" edge wouldnt' work, as the "bottom" doesn't stay on bottom for long!

I discovered that particles will bounce off objects that do not have "visible" checked, so whether the alpha trick or making the objects invisible, that's a great solution.

GKDantas, I suspect that this isn't a "simple" feature to add; as Mark mentioned, the code would have to be rewritten.  But I agree, a good fluid dynamics system would be most welcome, as would the 3D paint.

Those of us who have been here since RayDream and Infini-D may remember that RayDream used to have a 3D paint system.  What happened to that?  When MetaCreations was retooling RayDream and incorporating Infini-D archetecture, why did they drop the 3D paint?  It was slow on machines of the day, but I'd bet it would scream on a modern system....  Anybody know where it went?

:D