Take three shot glasses and place them on the bar in a row with the two outside shot glasses (the ones on the left and the right) rightside up and the middle glass upside down (see fig.1).

Bet your mark that you can put all three shot glasses bottoms-up in three moves, turning only two glasses in each move.

• First move: flip glasses 2 and 3
• Second Move: flip glasses 1 and 3
• Third move: flip glasses 2 and 3

All the glasses are now bottoms up. Let your mark try the trick. Set up the glasses as shown in fig.2

The mark won't be able to figure out why the glasses can't be put bottom up in three moves. It's because he doesn't realise that you've set the glasses up differently than you had set them up for yourself. Even if he copies your moves exactly, the glasses will only end up bottom down.