A simple little child's jigsaw puzzle, but with a little technological twist. The pieces fit into a base which contains a light sensitive diode, in the middle of a shiny metal tray. Any available light will bounce around the reflective metal surface, and because the diode will be able to detect the light, the action will not start. Only when the very last piece is in place, and the diode is in complete darkness will the circuit be activated, and the tune will play - a little reward for getting the puzzle right!