Exhibit: Pig Pen Puzzle
The wooden “fences” each have three slots in one side, like a comb. They fit together, a long slot in one, fitting a short slot in the other. The puzzle is how to use the set of slotted fences available to make the perfectly fitting pen.
The six fences are all different, and are the complete set of combinations of 3 short and long slots: LLL, LLS, LSL, LSS, SLS and SSS.
If you proceed by trial and error you will usually find that the last fence won’t fit!
Remove the “top” fences to turn a “bottom” one around and try again. There are 60 different ways in which the bottom three fences can be chosen, so a systematic search for solutions is a lengthy procedure!