A professor began his class by holding up a glass with some water in it. He held it up for all to see and asked the students, “How much do you think the glass weighs?” They retorted: “150 ml! … 200ml! … 250 ml!” the students answered. “I really don’t know unless I weigh it,” said the professor, “But my question is: “What would happen if I held it up like this for a few minutes?” “Nothing,” the students said. “Ok! What would happen if I held it up like this for an hour?” he asked. “Your arm would begin to ache,” said one of the students.