The first sliding block puzzle was invented in the 1870's. It was THE "15". America's greatest puzzle expert, Sam Loyd,"drove the entire world crazy" (as he himself put it) with this newly invented puzzle.
"THE 15" is one of the most interesting intellectual games in the
history of mankind" - these are Rubik's words and each of us will
certainly agree with them. However, while playing "THE 15" you had the
feeling of a certain incompleteness, didn't you? The reason is the vacant cell which,
indeed, should be filled in by the sixteenth block! But it is impossible, you will say,
the blocks will then loose their capacity to move! Yes, of course, if they are placed in
the box ...