This is magic RIGHT IN THEIR HANDS!
Show your spectator a small brass box that has a hole cut into the lid.Let him mark a coin (so that he is sure that the coin is never switched).Place the coin in the box, and screw the lid on.
Hand the box to your spectator and tell him to open it. HE CAN'T!!
Yet the moment the box is back in your hands, you can easily open it and take out his coin!
And you can use other items besides a coin!Use a finger ring, a piece of candy- anything that fits in the box!
The ring in the product photo does NOT come with this trick- it is in the photo to show you the size of this box.
You supply the ordinary coin.