Impossible Square - Wooden Puzzle Brain Teaser

This geometric construction originally discovered by Martin Gardner has all attributes of brain teaser. One can understand it only by a fine geometrical introspection; and even then, it is still surprising. Only four stars from me, because one could have done a square frame matching the pieces more exactly, and so could have achieved a stronger surprise by the customer. With two - three millimeters tolerance along all edges, I think that the effect was diminished.