Bootstrap Collapse not Collapsing

I was getting crazy for one hour... I reply to this question after 7 years, because if you are using the new Boostrap 5, the attributes are data-bs-toggle, data-bs-parent, and data-bs-target, pay attention to the -bs- in the middle.


jQuery is required (if you're using Bootstrap 3 or 4) ;-)

<html>
<head>
    <link rel="stylesheet" href="http://netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css">
    <!-- THIS LINE -->
    <script src="//code.jquery.com/jquery-1.11.0.min.js"></script>
    <script src="http://netdna.bootstrapcdn.com/bootstrap/3.1.1/js/bootstrap.min.js"></script>
</head>
<body>

<div class="panel-group" id="accordion">
    <div class="panel panel-default">
        <div class="panel-heading">
            <h4 class="panel-title">
                <a data-toggle="collapse" data-parent="#accordion" href="#collapseOne">
                    Collapsible Group Item #1
                </a>
            </h4>
        </div>
        <div id="collapseOne" class="panel-collapse collapse in">
            <div class="panel-body">
                Anim pariatur cliche reprehenderit
            </div>
        </div>
    </div>
</div>

</body>
</html>