Jack was angry. Joe owed him money. Joe owed him $100. Joe had borrowed $100 a month ago. He had borrowed the money 30 days ago. He said he would pay Jack back the next week. He said that he would pay Jack back in seven days. But he didn’t pay Jack back in seven days. He didn’t pay Jack back in 14 days, or 21 days, or 28 days. After 30 days, Jack was very angry. He called up Joe. “You owe me $100. You borrowed $100 from me 30 days ago. Where is my money? I want my money now,” Jack said. Joe said, “Oh, I’m so sorry. You’re right. I borrowed $100 from you. I owe you $100. Can I pay you back next week?”