Fascinating in it's simplicity, yet complex to solve
This puzzle consists of a transparent frame enclosing 8 identical coloured pieces arranged as if in a 3 by 3 grid. These pieces are three dimensional crosses, each with six perpendicular arms of different colours. The frame has three vertical slots at the front and three horizontal slots at the back through which the front and back arms of the pieces stick. There is one space in the 3 by 3 grid, and any adjacent piece can be moved into the vacant space, but the slots in the frame force a piece to roll over as it moves. By rolling the pieces around they get mixed up. The aim is to arrange them so that the space is in the middle and that the crosses all have matching orientation.