The professor holds up a glass of water to demonstrate to students how problems that seem small can become overwhelming. He asks students to guess the weight of the glass, which remains the same no matter how long he holds it. However, holding the glass for a long time would cause muscle strain and pain. Similarly, dwelling on life's problems heavily can stress and paralyze us, so it is important to "put the glass down" at the end of the day to avoid that stress building up.