The professor held up a glass of water to demonstrate to students how problems can cause stress over time. While the weight of the glass did not change, holding it for a long time would cause muscle strain and pain in the arm. Similarly, dwelling on life's problems for prolonged periods can lead to stress and inability to function well. It is important to think through challenges, but also critical to "put the glass down" at the end of the day to avoid being paralyzed by stress.