Kettlebells are cast iron weights, ranging from 5 lbs to over 100 lbs, shaped like a ball with a handle for easy gripping. The kettlebell originated in Russia and was popular in the U.S. decades ago, but has hit a resurgence in the last few years with a flurry of classes, videos and books. The reason? Kettlebells offer a different kind of training using dynamic moves targeting almost every aspect of fitness – endurance, strength, balance, agility and cardio endurance. People love it because it's challenging, efficient and you only need one piece of equipment.