求问大神，为什么我这么写，点了stop按钮再点start按钮再点stop按钮就无法停止了？查出来错误是吧加粗部分的var去掉，我就想不通，不去掉重新定义一个i难道clearInterval(i)就识别不出i了吗？求解释
<!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 i=setInterval("clock()",100);
function mystart(){
var i=setInterval("clock()",100)
}
</script>
</head>
<body>
<form>
<input type="text" id="clock" size="50" />
<input type="button" value="Stop" onclick="clearInterval(i)" />
<input type="button" value="start" onclick="mystart()" />
</form>
</body>
</html>
