a student of mine wants to use the jit.phys objects to place several small balls inside a hollow, transparent sphere. The small balls should roll around inside. At the moment we have a problem whereby they occasionally fall through the sphere. We are trying to understand exactly why this is happening. It doesn't seem to be related to the dimensions of the outer sphere mesh. Are there any phys settings we can tweak in order to make the large sphere totally non-porous?