If you only have 20 minutes to exercise, then we highly suggest that this is the workout you choose. Trainer Yumi Lee of Reebok CrossFit Lab will take you through an intense kettlebell workout that will not only work every part of your body, but is also sure to get your heart beating quickly too. Think of it as your go-to for building muscles and burning calories, all in one shot. For this workout, Yumi suggests using a kettlebell that weighs 20 pounds or less.