private function onMouseDown(event:MouseEvent):void
        {
        // story.text = "click: " + ball.x + "/" + ball.y;
        oldX = ball.x;
        oldY = ball.y;
        addEventListener(MouseEvent.MOUSE_UP, onMouseUp);
        ball.startDrag();
        removeEventListener(Event.ENTER_FRAME, frame);
        addEventListener(Event.ENTER_FRAME, trackVelocity);
        }
        
        private function onMouseUp(event:MouseEvent):void
        {
        stage.removeEventListener(MouseEvent.MOUSE_UP, onMouseUp);
        ball.stopDrag();
        removeEventListener(Event.ENTER_FRAME, trackVelocity);
        addEventListener(Event.ENTER_FRAME, frame);
        }
        
        private function trackVelocity(event:Event):void
        {
        vx = ball.x - oldX;
        vy = ball.y - oldY;
        oldX = ball.x;
        oldY = ball.y;
        }