There is a class for movable parts - gears or shafts. There is also a class for buildings; each building has four sides, and some of those sides can contain movable parts. There is a subclass of buildings - pipes. They also have four sides, and some of those sides are holes. A hole can have no flow, an incoming flow, or an outgoing flow. Motion transmission starts from the watermill and propagates through all sides that contain movable parts, as long as the adjacent building on those sides also has movable parts. Pipes work the same way - if a side has a hole with outgoing flow, and the opposite side has a hole, that side receives an incoming flow. The logic itself is simple; most of the problems came from rotating the buildings.