A professor holds up a glass of water in class and asks students to guess its weight, which they do. He then asks what would happen if he held it up for minutes, hours, or days. The students say his arm would get tired or go numb. Though the weight of the glass did not change, his arm was stressed from holding it. The professor explains that life's problems are like holding the glass - they seem manageable at first but become stressful over time. The important thing is to "put the glass down" by not stressing over problems when not actively working on them.