Step 1. The con man buys something for under $1. He gives the cashier a $10 bill.
Step 2. When the cashier hands back the change, the con says, “I really didn’t want all this change, and you probably need it.” He then offers the cashier ten $1 bills in exchange for the $10 bill he used to make his original purchase.
Step 3. The con takes the $10 bill from the cashier as he supposedly hands her the ten $1 bills. But the con actually hands her nine $1 bills and one $10 bill, for a total of $19, apparently mistaking a $10 bill for a $1.
Step 4. When the cashier points out the con’s overpayment, he acts surprised and thanks her for her honesty. If she fails to notice the error, he says that he might have made a mistake and would she please recount the money.
Step 5. Once the cashier is aware that she is now holding $19 instead of $10, the con immediately says, “Here, I’ll add another dollar to the $19 in exchange for a $20 bill. The con hands the clerk another dollar and takes the twenty. He then leaves the store with a $10 profit. (Of the $19 she was holding, $10 was hers. The scam was in acting as if it was all his.)
If you still don’t see the scam, go back and count it out, and be thankful you are not a cashier. The con has handed over $10 for the item and $20 in change—$30 in all—but he walks off with the first $9 in change, the $10 bill, and, finally, the $20 bill—$39—plus the item and the coins worth $1.