A simple one....from the Reader's Digest .

One afternoon a very preoccupied looking young woman got on my bus. About 15 minutes into the ride, she blurted out, "Oh, my gosh, I think I'm on the wrong bus line." I dropped her at the next stop and gave her directions to the right bus. "I don't know where my mind is today. I must have left it at work," she apologized. Just before she got off, I noticed she was wearing an ID card from an area hospital. "Are you a nurse?" I asked. "Oh, no," she said. "I'm a brain surgeon."