Hi lyes,

> Hi Michael if you shell sphere you get 2 spheres one
> inside the other and that is a Groop!!!!!!!!!!!

That's not exactly a group - the 2 spheres in that case actually make up 1 solid that has an internal cavity in it, like a hollow sphere.

The inner sphere in that case is the boundary of the interior void area.

If you do a boolean with a line to cut a sphere like that and then move the pieces you will more easily see that it is a solid with wall thickness.

- Michael