Sometimes known as Celts, or Wobble Stones, this is an ancient toy that has been fascinating people for a very long time. It is thought that originally people discovered that certain naturally occurring stones could show this property, if they had the correct shape. One can imagine people ascribing all sorts of mysterious powers to these stones originally!
Physics can now explain what is happening, although the equations are complex. But basically if you spin it anti-clockwise it will just spin normally. Try and spin it clockwise though, and the magic appears. It will slow down until it has completely stopped spinning. It will then 'rattle' or wobble along its length, and it will then spin a certain amount in the opposite direction.
Of course, we now know that it is just science, and not magic. Don't we??