@zed111: In general, I don't come by this chatroom very often, so you might try the regular chat. But I'm going to suggest a resource to you that may be helpful. Shawn Walker recently wrote a book called The Shape of Things, and your question is very much a question about shape geometry (moving shapes).
The simple answer to your question, however, is that we're looking for extrinsic motion of the surface as time passes. If interior particles move tangentially along the surface, that doesn't change the overall surface as we view it. But moving normal to the surface or having the edge of the surface move clearly changes its overall position in space.
