A good example would be a children's book I used to have when I was younger. It was called "Windows" I can't find any pictures of it but it was set up like this:
The pinwheel type mechanism would have one image layered under another. When you pulled the hoop it would slide the triangles of image (a) under image (b), then back when you pulled it again.
But this idea could be portrayed in a number of different ways as well. As an open-ended idea it allows this to be interpreted in a multitude of ways. I think it would be interesting to see the turn out.