A problem with a randomly coloured background

I use StackOverflow for the first time, so please be friendly and understanding.

Few days ago I got interested in ActionScript. I have downloaded FlashDevelop (a free IDE) and FlexSDK4. Then I have learned the basics from some tutorials.

For now I am not really developing any big project, I'm rather just doing tests.

Anyway, a solution to my problem is really important for me. I have looked for it, but I couldn't find any.

I have just one button and a background in my app. Both the button and the background (below: "bg") are objects of the Sprite class.

When I click the button, the background gets painted with 10x10px squares of random colours. The problem is that the more times I click the button, the longer time I have to wait until the background changes. And that's not all! I can change the background exactly 54 times! At the 55th time it doesn't change at all.

package {
    // some imports here

    public class Main extends Sprite {
        private var button:Sprite;
        private var bg:Sprite;

        public function Main ():void {
            init();
        }

        private function init (e:Event=null):void {
            addChild (bg);
            // in the original code there are some lines here,
            // in which the button is created
            addChild (button);
            button.addEventListener (MouseEvent.CLICK, btnClick);
        }

        private function开发者_StackOverflow中文版 btnClick (event:MouseEvent):void {
            var x:uint, y:uint, color:uint;
            for (y=0; y<30; y++) {
                for (x=0; x<40; x++) {
                    color=Math.round(Math.random()*16777215);
                    bg.graphics.beginFill (color);
                    bg.graphics.drawRect (x*10, y*10, 10, 10);
                    bg.graphics.endFill ();
                }
            }
        }
    }
}

The code is so short, because I have removed many void lines. I have left only the important ones.

What is wrong with this code? Please help me.

Thanks in advance.

clear your graphics before the loop

 var x:uint, y:uint, color:uint;
 bg.graphics.clear();
 for (y=0; y<30; y++) {//etc