In the 1950s and 1960s inventive engineers saved space by using the 'cordwood' assembly, where components are sandwiched in between two circuits boards. This construction became less useful with reduced component sizes, PCB manufacturing, and surface mount technology, but is still a lovely piece of history. Combining both old and new, the Cordwood Puzzle is a tribute to this construction.
The puzzle is to correctly assemble the circuit with the components supplied. Once completed, all LEDs light up when power is applied, and when connected to a controller board each LED can be individually controlled. (When connecting to a board, make sure that the I/Os can tolerate the voltage levels that are applied to the Cordwood. Also, make sure that you do not short power and ground!)
Unfortunately(!), all the comprehensive assembly guides are mysteriously missing from the kits (but a surviving copy may be found here).