Tryit Editor v1.8

Source Code:

<!DOCTYPE html>
<html>
<head>
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.1/jquery.min.js">
</script>
<script>
$(document).ready(function(){
  $("button").click(function(){
    $("p").fadeTo(2000,0.2,function(){
      alert("The fadeTo effect is finished!");
   });
  });
});
</script>
</head>
<body>

<button>Gradually change the opacity of the p element</button>
<p>This is a paragraph.</p>

</body>
</html>

Result:

W3Schools.com - Try it yourself

Edit the code above and click "Submit Code" to see the result.