<html>
<head>
<title>example</title>
</head>
<body>
<div style="width: 100px; height: 100px; background-color: red;"></div>
<script src="http://code.jquery.com/jquery-1.10.2.min.js"></script>
<script>
var div = $('div');
$(div).hover(function ()
{
$(this).stop().animate({opacity: 0}, 500);
}, function ()
{
$(this).stop().animate({opacity: 1}, 500);
})
</script>
</body>
</html>