I was standing in the grocery store trying to decide whether I could afford cereal when an old man walked up beside me. He looked at my cart. Then he looked at me. Then he said, “Put the cereal back.” I thought he was joking. “I beg your pardon?” “Put it back.” I laughed. He didn't. “You're trying to save money, aren't you?” I nodded. “Then don't spend money just because you're tired.” That sentence hit me harder than it should have. I put the cereal back. The man smiled and walked away. A few minutes later, I reached the checkout. The cashier scanned my groceries. Then she said, “Someone already paid.” I looked around. The old man was standing near the entrance. I ran after him. “Sir! You paid for my groceries?” He shrugged. “You needed them.” I tried to give him the money. He refused. Then he said something I'll never forget. “When I was your age, someone helped me. I promised myself I'd do the same whenever I could.” I asked him who had helped him. He smiled. “My wife.” He pulled an old photograph from his wallet. It was a picture of a young woman standing outside a grocery store. I looked at it. Then I looked at the store. It was the same store we were standing in. He told me she'd passed away years earlier. But before she died, she made him promise that he'd never stop helping people. “So I don't,” he said. I thanked him and went home. A few weeks later, I was back at the same grocery store. I saw a teenager standing in front of the cereal aisle, staring at the prices. I remembered the old man. So I bought the cereal for him. He looked shocked. “You don't have to do that.” I smiled. “I know.” And as I walked away, I realized something. The old man hadn't really bought me groceries. He'd passed something on. And now it was my turn.