Toobz/Pipes/Plumber is a real-life version of the online flash/java games of the same name. Through playing the game the user gains an understanding of flow, and has a fun time wih water.
The main problem with this toy is water-sealing it, so that the pipes can actually function as pipes, aslo making sure that pipe-pieces cannot be swallowed. This pipe needn't be too expensive though, as it can mostly be made out of cheap plastic. Managing drainage/overflow are also issues that need to be addressed