I've heard lots of talk about hollowing but what does this actually mean? If I upload a normal shape, e.g. a cube, Shapeways thinks it's solid, and prints the material right through - is that correct? So instead I need to upload what's effectively a box instead of a cube - with internal walls - hence making it hollow. And then Shapeways prints it as a box and it's hollow. But then I hear talk about leaving holes and so on. So I'm very confused. Do you need to leave a hole? What's that for? Thanks anyone who can help.