MouseEvent (extreme newbie)

An extremely basic question I’m sure but I can’t figure out why this is so.
Why does this only work when I click on the ball, and not when I click anywhere on the stage?

public class Main extends MovieClip {
    public function Main() 
    {
        addEventListener(MouseEvent.MOUSE_DOWN, click);        
    }    
    private function click(e:Event):void
    {
        ball.x = mouseX;  // ball is an instance
        ball.y = mouseY;
    }    
}