去掉注释后可以弹出alert但是点击按钮后没有打开新窗口，求指点
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>window对象</title>
<script type="text/javascript">
//alert("欢迎来到慕课网")
function open(){
window.open("http://www.sina.com","_blank","width=600,height=400")
}
</script>
</head>
<body>
<form>
<input type="button" value="点击我，打开新窗口" onclick="open()" />
</form>
</body>
</html>
