Fade out onrelease

Ok. I need some actionscript help. I have a movieclip called ball. the instance name is ball. I also have a button. This is all on Scene 1. What I want is that when I release the button the ball should start fading out.
I tried this code:


on(release) {
	var speed = 10;
	_root.ball._alpha -= speed
}

But when I try this out every time I click the button the ball’s alpha goes down by 10 and I have to keep on clicking. WHat should I do that when I click the button the box just fades out by tens.