A professor holds up a glass of water in class and asks students to guess its weight. When the students provide guesses, the professor notes he doesn't actually know the weight unless he measures it. He asks what would happen if he held the glass up for minutes, then an hour, then a day. The students note his arm would begin to ache and could become numb. The professor explains that while holding the glass, its weight did not change - the stress came from holding it. He tells the class that problems are like the glass: they seem manageable at first but can paralyze us if dwelled upon. The most important thing is to "put the glass down" by not stressing about challenges at the end