Merlin's Magic Potion

From a beautifully decorated precious wood box, you remove 10 marbled wooden blocks, which are arranged together to form a rectangle of 14 x 12 cm, and around which a frame fits perfectly: a mysterious puzzle.

For example, you could say: "When I bought this puzzle, the salesman persuaded me to also buy this liquor... (showing a filled shot glass) ...and he assured me that it was an original Merlin potion, and without this potion, no one would be able to arrange the puzzle correctly. So I bought the potion too—and tried it out right away at home...!"
You take the individual puzzle pieces and put them together; and lo and behold, it works perfectly, without problems and without a magic potion.

"I went back to the store and said, 'It works without a magic potion!'" The salesman said, 'Indeed, I forgot to give you another piece of the puzzle!'
You put the puzzle together again and this time you succeed, even faster than before – and again without any magic potion.

"So I went back to the salesman and said, 'You don't need the magic potion at all,' to which he replied, 'Sorry, I actually forgot to give you this extra part!'

And once again you put the puzzle together and again all the pieces fit together perfectly, and again without a magic potion...

“What did you sell me the magic potion for?” you asked the seller.

"Look," he said calmly, "you've added two elements to the puzzle - and yet it has exactly the same dimensions as it did at the beginning" (you place the frame around the rectangle and it fits exactly around it, just like at the beginning), and to understand that, you need a sip of the magic potion!" Cheers!

An inexplicable miracle that will leave viewers wondering! For a long time!

You already have the glass and the magic potion. We'll provide everything else. One more thing: the more intelligent the audience, the more they'll puzzle over this miracle!

The secret of the trick is more sophisticated than you might think. You'll like it.