Sometimes an object looks like a solid, but in fact it is just a "Joined srf". In my words the latter means, if you pour water into it, it wouldn't stay in there. But can MoI show me where the water would flow out? And when I join two surfaces whether that connection could be part of a solid?

Hi Strube, set up a keyboard shortcut with the script shown here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=6051.2

That will then give you a way to see "naked edges" in your model - those are edges that belong to only one face instead of being joined between 2 faces. Those are the edges that will prevent a model from being a fully closed and watertight solid.

- Michael

Works great. Thank you.

Now that I know they are called "naked edges", the snippet is also on Petr's site. It is called "SelectNakedEdges".