private function copy(event:TimerEvent) {   
       
    var mc = new Fly(Math.random() * 10 + 1);   
    mc.x = Math.random() * this.stageW;   
    mc.y = this.stageH;   
       
    content_mc.addChild(mc);   
    mc.addEventListener(MouseEvent.ROLL_OVER, downHandler);   
    mc.addEventListener(Event.ENTER_FRAME, removeDrop);   
}   
  
private function downHandler(event:MouseEvent) {   
       
    var mc = event.target;   
    mc.removeTimerHandler();   
    mc.removeEventListener(MouseEvent.MOUSE_DOWN, downHandler);   
    mc.removeEventListener(Event.ENTER_FRAME, removeDrop);   
    content_mc.removeChild(mc);   
       
    //refreshGrade(mc.flySpeed);//還可以通過擴展refreshGrade方法，按不同速度得分   
    refreshGrade();//按數量   
       
}   
  
private function removeDrop(event:Event) {   
    var _mc:MovieClip = event.target as MovieClip;   
       
    if (_mc.y <= 0) {   
        _mc.removeTimerHandler();   
        _mc.removeEventListener(MouseEvent.MOUSE_DOWN, downHandler);   
        _mc.removeEventListener(Event.ENTER_FRAME, removeDrop);   
        content_mc.removeChild(_mc);   
    }   
       
}   
  
public function refreshGrade(grade:Number = 1):void {   
    this._grade += grade;   
    displayGrade_txt.text = this._grade.toString();   
}