There's nothing magical about the number 10,000. In fact, the idea of walking at least 10,000 steps a day for health goes back decades to a marketing campaign launched in Japan to promote a pedometer. And, in subsequent years, it was adopted in the U.S. as a goal to promote good health. It's often the default setting on fitness trackers, but what's it really based on? "The original basis of the number was not scientifically determined," says researcher I-Min Lee of Brigham and Women's Hospital. She was curious to know how many steps you need to take a day to...