1. A jigsaw toy comprising a plurality of central pieces and edge-defining pieces having first and second faces, at least some of the pieces having cooperating narrow mouthed sockets and enlarged heads for enabling the pieces to be interlocked to form a coherent shape; at least one of said edge-defining pieces being asymmetrical and removable from a first position in which said first face is displayed and is reversible to display said second face, and then replaceable in said first position, said at least one edge-defining piece also being removable from said first position to a second position, said replacement of said at least one edge-defining piece in said first position and movement of said at least one edge-defining piece to said second position altering the visual effect of said coherent shape without substantially altering the outline of said coherent shape, at least some of said pieces having indicia on at least their first faces such that intelligible composite designs are formed when said first piece displays both its first and second faces in said first position and when said first piece is disposed in said second position.