With a soup can or other light object in each hand, hold our arms out to your sides with your elbow bent 90 degrees so your upper arms are just an inch or two below your shoulders and almost parallel to the floor, and your forearms are pointed in toward your torso.
Keep your upper arms still and use your elbows to rotate your forearms up until they are as close to perpendicular to the floor as possible. Return to the starting position.