A professor holds up a glass of water in class and asks students how much they think it weighs. When students provide guesses, the professor notes he doesn't actually know unless he weighs it. He asks what would happen if he held it up for minutes, then an hour, then a day. While the weight wouldn't change, holding it for longer periods would cause muscle strain and pain in his arm. He tells students this is like problems in life - thinking about them for a short time is manageable, but dwelling on them for too long can paralyze you. The lesson is it's important to think about challenges, but more important to "put the glass down" or stop dwelling on problems at the end of each day