(function($){
	window.addEvent('domready',function(){
		//key var
		var opacity = 0.5, toOpacity = 1;
			
		//set opacity ASAP and events
		$$('img.btn').set('opacity',opacity).addEvents({
			mouseenter: function(){
				this.tween('opacity',toOpacity);
			},
			mouseleave: function(){
				this.tween('opacity',opacity);
			}
		});
	});
})(document.id);