提交代码说对了，但是点击停止按钮没效果，求指点
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>计时器</title>
<script type="text/javascript">
function clock(){
var time=new Date();
document.getElementById("clock").value = time;
}
var s=setInterval(clock)
</script>
</head>
<body>
<form>
<input type="text" id="clock" size="50" />
<input type="button" value="Stop" onclick="clearInterval(s)" />
</form>
</body>
</html>
