I think I've got it. But not completely. I made this image and placed the primitives in place with the move tool as I made them. Some closer and some not so close. But I couldn't get any far away that wasn't floating.
I rendered the image and they were all sitting on the ground. I then went back to the wireframe and added another sphere and that one wouldn't sit on the ground unless it was close to the front. I tried to place the smaller sphere further back and as you can see it's floating. I did all the moving with the move tool.
So what am I missing here. I re-read all the 5 parts of this weeks lessons a few times and still don't get it.