Some people like to dangle off cliffs.
Others kayak down killer rapids. In this hour of To the Best of Our Knowledge , why people are driven to push the limits. Also the story of Jerri Nielson, the doctor stranded at the South Pole who discovered she had breast cancer, and then treated herself. Environmental writer Bill McKibben tells Jim Fleming about his decision to push himself to become a world class cross country skier. He explains that his training took a year, during which he also had to cope with his father's death.