The professor holds up a glass of water in class and asks students to guess its weight, which varies, but all agree it does not change. He then asks what would happen if he held it up for longer periods - his arm would ache after an hour and go numb after a day. Though the weight did not change, the stress of holding it up for long periods is what caused the issues. The professor concludes that problems in life are similar - they seem manageable at first but holding them in your head for too long can paralyze you. It is important to "put the glass down" by not stressing about problems before sleeping, so you can handle challenges fresh the next day.