[AccessD] Design shenanigans

I have a small design problem that I thought I might throw out there to see if anyone has a solution:

It goes like this - I have the following tables:

Projects
....Sub Projects
..........Activities

The problem is that not all Projects have Sub Projects so the Activities could be linked directly to the projects table as well as the Sub Project table.

My initial thoughts are that each project would need at least one sub project - i.e. the 'forced' sub project is used only to link over to the project table. 

Now that I think a bit more, I'm not really liking that idea.

What to do .... what to do?

Anita Smith