Take a piece of paper out of a bag, cleanly hand it to the spectator.
You have a little contest in free choice. Have them mix up e disks, each has a different shap on one side. They lay face down on the table. THEY choose one disks for you & one for them & puts inside the bag.
ALL SEEMS FAIR SO FAR right?
They open the paper from earlier thats been in plain view, unfold it...........