Start seated side by side with a weighted medicine ball, hand weight or large phone book (anything that weighs about 6-8 pounds).Lean slightly back off your "sits bones" onto the soft part of your backside and pass the weighted object to your partner. Lift your arms over head and lift one leg (keep both feet on the ground if you need to). Pass the weight back to your partner. Try for 8 to 10 reps with your partner on your left, then switch sides. This move borrows Pilates core moves with weight-training waist rotations, which results in a tight midsection and great shoulders.