(function($){
	window.addEvent('domready',function() {
		//element collection and settings
		var opacity = 1, toOpacity = 0.5;

		//set opacity ASAP and events
		$$('div.opacity').set('opacity',opacity).addEvents({
			mouseenter: function() {
				this.tween('opacity',toOpacity);
			},
			mouseleave: function() {
				this.tween('opacity',opacity);
			}
		});
	});
})(document.id);