Casino owners have the nerve to insist that players asked for years for the abolishment of coin-operated slots. They say gamblers were getting dirty hands and it was too hard to carry those coin cups to the cashier's cage for reimbursement. Now, the paper receipts, I was told, use a light-sensitive ink made to disappear if uncashed a while after play. The ink might also fade if touched as skin has an oily base. I know some folks are collecting these receipts, good luck !