You'll love this beautifully crafted mentalism item! Three crocheted balls of different color sit atop the closed box. While your back is turned, the spectator selects one of the balls and shows it around for the others to see. He places it right back where he got it and tells you it is OK to turn back around.
Have the person open the lid of the box and place the balls inside in any order they wish. Close the lid. Explain that before you began the experiment in mentalism you placed a duplicate ball in your pocket. Hopefully it matches the person's free choice. Remove it from your pocket and hold it in your fist. He names the color. Open your hand and it matches. In fact, it is actually his ball! Have the spectator open the box. Their ball is missing!
No duplicates are used. Clever and practical!