The professor holds up a glass of water to demonstrate to students how problems that seem small can become overwhelming if dwelled upon for too long. While the weight of the glass did not change, the professor's arm would begin to ache after holding it for an hour and could become numb or experience muscle stress if held all day. This represents how small problems felt in one's head can start to negatively impact you the longer you think about them. It's important to acknowledge challenges but also to "put the glass down" or stop thinking about problems at the end of the day so you can wake up fresh and able to handle what comes your way.