What to see
Observe the triangle above. It consists of 4 coloured pieces, and a missing square. Now press the “re-order pieces” button. Three pieces move to another place, the overall shape seems the same, but the square is no longer missing!
What to do
Repeat the re-arrangement of the pieces and have a go at the puzzle. Where is the missing square? What happens here? Be assured that this is no computer trick, it would work just as well with pieces cut from cardboard. In fact, it’s great fun to do so and make a nice puzzle for your dear one(s).
Martin Gardner (1965) Mathematics, Magic and Mystery, New York, Dover Publ.
Good explanation at Grand Illusions
Ok, ok, here’s the solution.