Put the plastic cup down

      The professor hold a plastic cup full of water in hands before class. He hold up plastic cup to let all students see it, and asked: ”what happen if i hold it up for couples of minutes?” 
     

clear plastic cup

      “nothing” the students said
      “yes, you are right,how about the whole day”
      “Your arm will go numb,and you might have sever muscle stress”a student suddenly said
      “OK, nevertheless, did the wight of plastic cup change?” the professor asked.
      “No” all students replied.
      “what made you feel pain” the professor asked "and what should i do before my arm feel pain”.
      All the students are confused.
   

plastic lemon cup

      “Just put plastic cup down” there is a students replied
      “Absolutely” the professor said, ”Just like our life, it is important to consider the question and challenge of your life, but most important is putting down all things you have thought in day, then you may have more energy to respond efficiently to new challenges and opportunities.