Imagine having a card selected and a corner torn and then you are able to take the piece and melt it through the back your clear solid plastic phone case ! You then can match up the missing piece that's now on the other side of your phone case with the card that was selected to prove in fact that it is indeed the same piece you ripped off from the selected card !

And as a bonus effect you can do this with a dollar bill and show that the serial numbers exactly match and the piece fits perfectly

This is simple and easy to do

Super visual magic

Everything can be examined at the end

They can keep the corner and selected card as a souvenir