Arrange three cups in a line on a table. The two cups on the ends should be mouth down, while the middle cup is mouth up.
Tell your audience that you are going to make all the cups end up mouth up. Also, instruct them to pay careful attention so that they can do it too.
Start with the middle cup and one of the end cups. Turn them both over. Turn over the two end cups. Finally, turn over the two cups that you started with. Now, all of the cups will be mouth up.
Ask your audience if they paid attention and if they are ready to try. Remind them they have to do exactly what you did. Of course, they will say yes.
Turn over only the middle cup so that it is mouth down. Now, the arrangement of the cups is exactly opposite from where they were when you started, but hopefully your audience has not noticed this. This is a sort of an optical illusion, and your goal is to make the trick fast paced and do a lot of talking so that they don't notice what you have done.
Instruct your audience to turn over the same cups that you did, and in the same order. When they do this, the cups will all end up facing mouth down. The secret to this trick is that the position of the middle cup will determine where all the cups will end up, providing that the outside cups are not in the same position.