This cross-lateral exercise sculpts your arms and fires up your abdominals in order to stabilize your body.
How to do it: Stand with feet together (or make it easier by standing with feet hip width), knees slightly bent, holding dumbbells (heavy enough to fatigue your arms in 10 repetitions). Engage your abs and keep your body as still as possible as you curl your right arm across your body towards your left shoulder, palm facing up. Lower arm and repeat with left arm. That's one rep. Try up to 3 sets of 10 repetitions.