Give me your coins...and I will give you some coins back

I went the bank the other day to cash a check. Carter came with me. He had 15 cents in his pocket and he was just walking around the bank. Well, he found one of those coin deposit machines. So, he deposited the two coins he had into the machine and it gave him a receipt for 15 cents. He looks at me and asks 'What do I do with that?' I told him that we could take it to the teller and give it to her and she would give us 2 coins back. He looked confused and said 'Well, thats kind of dumb.' Yeah, not much I could say to that except that next time he uses the coin deposit machine, we should use more coins.