jquery: what does "$(this)" exactly mean?
<p>I have a programme and it works well.
See <a href="http://jsfiddle.net/HP8Xd/" rel="nofollow">HERE</a>.</p>

<p>This is the code.</p>

<pre><code>&lt;div id="round"&gt;&lt;/div&gt;

&lt;style&gt;
#round{
position: absolute;
width: 200px;
height: 200px;
border-radius: 50%;
left: 400px;
top: 200px;
background-color: #e1e1e1;
}
&lt;/style&gt;

&lt;script src="jquery.js"&gt;&lt;/script&gt;
&lt;script src="jquery.easing.1.3.js"&gt;&lt;/script&gt;
&lt;script&gt;
$(document).ready(function(){
    $("#round").click(function(){
        setInterval(function(){
            $("#round").animate(
                {height: 250,
                width: 150,
                top:150,
                left: 425},
                {duration: 300}
                ).
            animate(
                {height: 200,
                width: 200,
                top:200,
                left: 400},
                {duration: 300}
            );
        }, 0);
    });
});
&lt;/script&gt;
</code></pre>

<p>But when I change "#round" to "this". It won't work. why?
(actually it works, but when I put them into setInterval(), it won't work)</p>

<pre><code>$(document).ready(function(){
    $("#round").click(function(){
        setInterval(function(){
            $("#round").animate(
                {height: 250,
                width: 150,
                top:150,
                left: 425},
                {duration: 300}
                ).
            animate(
                {height: 200,
                width: 200,
                top:200,
                left: 400},
                {duration: 300}
            );
        }, 0);
    });
});
</code></pre>

<p>change to "this", it won't work.</p>

<pre><code>$(document).ready(function(){
    $("#round").click(function(){
        setInterval(function(){
            $(this).animate(
                {height: 250,
                width: 150,
                top:150,
                left: 425},
                {duration: 300}
                ).
            animate(
                {height: 200,
                width: 200,
                top:200,
                left: 400},
                {duration: 300}
            );
        }, 0);
    });
});
</code></pre>
