Imagine an exercise that works every major and minor muscle group, burns 600 calories an hour, builds balance and flexibility, requires no investment beyond $25 to $35, and makes you feel exuberant, delightful, sassy, and sexy. And is so much more like play than work that you want to keep doing it. That's right, what goes around comes around--the Hula-Hoop, which burst into fad-dom fifty years ago, is hot all over again, but this time for enormously...