A "switch board" with four colored LEDs on plugs that fit into matching colored sockets and four switches under them. You demonstrate that only the switch under the plug (of the same color) is the one that controls the LED of similar color. Then interchange the LEDs (plugs) at random. The switch of any color still controls only the LED of the same color, and is the only one that does so, regardless of the socket it is plugged into. You'll have a lot of fun with this one!