You are in the checkout line at the grocery store when your eyes wander over to the ice cream display. It is a hot day and you could use something to cool you down before you get into your hot car. The problem is that you have left your checkbook and credit and debit cards at home—on purpose, actually, because you have decided that you only want to spend $20 today at the grocery store. You are uncertain whether or not you have brought enough cash with you to pay for the items that are already...