Hi lyes,

> Grooping 2 solids to make one solid is that what you saying

Yup, but it's more than just a simple group because in this case the inner sphere knows that it is the boundary surface of the interior hollow region.

It's kind of like how you can take surfaces that touch at edges and join them together to make a connected solid - in a certain sense that is a type of grouping mechanism but it's more than just a simple group because there is additional connection information created with edges being joined to one another.

The hollow sphere case is similar to that, it's not just simply 2 surfaces in a group, it's a structural arrangement where there is a solid region that knows that it has the 2 surfaces set up as the boundaries of the region, making a kind of wall-thickness to the solid with a hollow empty area inside of it.

- Michael