Doctor , I slipped and fell on my way to school . The ground is so slippery from the snow . It seems I can't move my left arm now .   Let me see . Roll up your sleeve , please . Um ... it's swollen and red here . I'm afraid we'll need to take an X-ray to see if it's broken .   What if it's broken ?   Then we'll put you in a cast .   What do you mean by a cast ? 