Merlin’s Magic Elixir by Harold Voit, sold by Steven's Magic Emporium
Effect: You remove 10 marbled blocks of wood from a beautifully decorated hardwood box. The pieces form a rectangle 14 by 12 cm, around which a white frame fits exactly-a puzzle. Then you tell the spectators: “When I bought this puzzle, the salesman also persuaded me to bug this potion, which was far more expensive than the puzzle, itself. ” …You show a bottle containing a liquid. “He assured me that it was the original magical elixir which Merlin, himself, had concocted. Then he said that without this elixir, no one could ever figure out the puzzle. So, I also purchased the elixir!” And then you attempt to reassemble the puzzle. And look, it works perfectly. It goes off without a hitch, and without the use of the magical elixir. Infuriated with the “tricky salesman” you go back into the store with the puzzle, and angrily place it on the counter and complain that you were able to solve it without using the elixir.
The salesman says: “Please calm down. I forgot to give you one of the pieces of the puzzle!” When you get home again, you try to assemble the puzzle again, and you succeed once more, even more quickly- again without the help of the elixir. So you take the puzzle back to the salesman once more, this time, even angrier than the time before. But he calms you down again. “I’m sorry, sir. I actually forgot to give you this piece, too!” And once more, you put he puzzle together again, right there in the store, right in front of the salesman’s eyes, and once more, the pieces fit ¡Together–¡perfectly. “Why…” you ask the salesman, “did you sell me this magical elixir with the puzzle?” “Look,” he replied, unperturbed. “You have added two pieces to the puzzle-and it is still exactly the same size that it was when you started (then place the white frame around the rectangle, and it fits exactly as it did as the beginning). And in order to figure that out… you will need to take a drink of the magical elixir!” Down the hatch!