Place 13 coins on 12 pieces of paper so that each piece of paper has only one coin on it, but all 13 coins are used. Give it a try!
Place the first coin on the first piece of paper.
On the same piece of paper temporarily place a second coin that we won’t–we think–be able to find a place for.
Now place the third coin.
Twelfth–and one piece of paper remains!
On the last piece of paper we place the thirteenth coin–the one we didn’t think we’d find a place for. So, we managed to place all the coins after all–with only one coin on each piece of paper.
Obviously, you see that this is a trick–we counted the second “temporary” coin twice. But try showing this to your friends and you’ll see how easily they are fooled, as long as you speak confidently and quickly–but not hurried. And while they stand around trying to figure out how you did it, gather up the coins before they can be counted.