Remember this painting?
This is the famous optical illusion painting from Dutch artist M.C. Escher.
Japan actually made a 3D version of this physically impossible sculpture.
This 6" sculpture is available at ThinkGeek for US $59.99.
Unfortunately, this is not a real waterfall. Furthermore, you have to view this sculpture at a certain angle. Otherwise, the whole illusion will be destroyed.
Well... I guess humans are not technologically advanced enough to bend the Law of Physics yet...