<!DOCTYPE html>
<html>
<head>
<script>
function selectMultiple() {
document.getElementById("mySelect").multiple = true;
}
</script>
</head>
<body>
<form>
<select id="mySelect" size="4">
<option>Apple</option>
<option>Pear</option>
<option>Banana</option>
<option>Orange</option>
</select>
<input type="button" onclick="selectMultiple()" value="Select multiple">
</form>
<p>Before you click "Select multiple", you cannot select more than one option (by holding down the Shift or Ctrl key).</p>
<p>After you have clicked "Select multiple", you can.</p>
</body>
</html>