You show a playing card that has a triangle cut out in the center you show that you can see the triangle from both sides of the card you then flick the card with your finger and the triangle instantly changes into a square you then explain to your spectator that not only does the triangle and possibly change into a square but it only changes on one side of the card you then turn the card over to show that on the other side of the card it's still a triangle which seems impossible to your spectators that you have a card with a square cut out on them one side and a triangle on the other then you were able to turn the square back into a triangle and show that the card is back to being an ordinary card with a triangle visible on both sides this effect is easy to perform it's visual and impossible looking