An interlocking puzzle that assembles into many different shapes. A very popular Stewart Coffin design.
The puzzle pieces are based on a particular dissection of a rhombic dodecahedron into identical halves, then two halves are joined together in all possible ways. There are 12 possible pieces, and this set of 6 was selected by Stewart Coffin as the best. For more information on possible shapes that can be constructed see this page on Puzzle World.
George Bell converted the design using spheres rather than rhombic dodecahedra. He discovered that some shapes can be assembled using the sphere pieces which are impossible for the wood versions. For example, the large triangle cannot be assembled using the wooden version of the puzzle, but it can be assembled using the Shapeways plastic version.