We threw everything but the kitchen sink at you in our first three workouts, getting you into the swing of using your bodyweight and testing your endurance. Now it's time to get familiar with a new piece of equipment: the kettlebell, essentially a cannonball with a handle.Kettlebells are the ultimate weapon for improving our cardiovascular endurance, power, and strength. A kettlebell swing uses the muscles of the hips, glutes, and hamstrings — with some help from our lats, abs, shoulders, pecs, and forearms (see why we're using them today?). Take yourself through our no-weights warm-up, then we'll go through our kettlebell workout.And just in case you don't have access to a kettlebell, we'll teach you how to modify the moves with a dumbbell. You'll get the same benefits. You just won't look like a strongman from an old-timey circus.When you're done with this lesson, check out Getting Fit from Scratch: Chest, Arms, and Shoulders in One Workout.