how can I define a variable in jquery and put a element on it?
what's the rule?
i'm new in jquery
my html code is 

```
<head>
    <title>Simplify, Simplify</title>
    <script type='text/javascript' src='script.js'></script>
</head>
<body>
    <div> Remember!
        <ul>
            <li>
                <ol>
                    <li>Start with the function keyword</li>
                    <li>Inputs go between ()</li>
                    <li>Actions go between {}</li>
                    <li>jQuery is for chumps!</li>
                </ol>
            </li>
            <li>Inputs are separated by commas.</li>
            <li>Inputs can include other functions!</li>
        </ul>
    </div>   
</body>

```

and my jquery is:

```
var target = $("#li:nth-child(4)");
$target.fadeOut('fast');

```

i want to fade #4 line in list order