Create an automatic expanding MultiLine TextBox like Twitter using jQuery in ASP.Net

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript">
    $(function () {
        $('#txt1').focus(function () {
            $(this).animate({ height: 100 }, 500);
        });
        $('#txt1').blur(function () {
            $(this).animate({ height: 30 }, 500);
        });
    });
</script>
<textarea id="txt1" rows="3" cols="12" style="height: 30"></textarea>