When I first graduated from college, I found myself needing to adopt a strict grocery budget—twenty-five dollars a week, to be exact. One day, while entering the checkout line, I suspected the groceries I’d selected cost slightly more than my remaining money. “Just stop when we reach twenty dollars,” I told the cashier, and was able to purchase everything I’d selected but a bag of peppers.

As I was about to drive home, a man stopped by my car. “Here’s your peppers, ma’am,” he said, handing the bag to me. Before I had time to thank him, he was already walking…