AS3 - Scale from center?

programming flash
1

Each time the square is clicked it scales up a little. What must be added or changed to prevent it from moving off center from its original position, and to have it scale evenly in all directions from its own center?


package {
    import flash.display.Sprite;
    import flash.display.Shape;
    import flash.events.*;

    public class NewTest extends Sprite{
        
        private var s:Shape;
        private var sHolder:Sprite;
        
        public function NewTest(){
                sHolder = new Sprite();
                addChild(sHolder);
                s = new Shape();
                s.graphics.beginFill(0xFFFFFF);
                s.graphics.drawRect(20,20,10,10);
                s.graphics.endFill();
                sHolder.addChild(s);
                sHolder.addEventListener(MouseEvent.CLICK, start);
            }    
        
        public function start(e:MouseEvent):void{
            s.scaleX+=1;
            s.scaleY +=1;        
        }
    }
}

Fingers