^A psychologist walked around a room while teaching stress
  management to an audience. As she raised a glass of water,
  everyone expected they'd be asked the "half empty or half full"
  question. Instead, with a smile on her face, she inquired: "How
  heavy is this glass of water?"

  Answers called out ranged from 8 oz. to 20 oz.

  She replied, "The absolute weight doesn't matter. It depends on
  how long I hold it. If I hold it for a minute, it's not a
  problem. If I hold it for an hour, I'll have an ache in my arm.
  If I hold it for a day, my arm will feel numb and paralyzed. In
  each case, the weight of the glass doesn't change, but the
  longer I hold it, the heavier it becomes."

  She continued, "The stresses and worries in life are like that
  glass of water. Think about them for a while and nothing
  happens. Think about them a bit longer and they begin to hurt.
  And if you think about them all day long, you will feel
  paralyzed