When I was growing up, I stayed with my grandparents for a week or two every summer. They lived on a street that dead-ended into some railroad tracks. I would often awaken several times on my first night as the box cars rumbled by or when an engineer blew the train whistle. By the end of my visit, however, I had grown so accustomed to the noise that I could sleep straight through the night without interruption. I had tuned out the sounds.