The professor held up a glass of water in class and asked students to guess its weight. When they said holding it for a long time wouldn't affect the weight, the professor asked what would happen if he held it for an hour or day. The students said his arm would ache or become numb. Though the glass's weight didn't change, the stress of holding it for a long time caused pain. The professor explained that problems are like the glass - they seem manageable at first but holding them in your mind for too long can paralyze you. It's important to 'put the glass down' by not stressing about challenges at the end of each day.