//开始不跑，原因是函数必须放在调用函数之前！！！
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>练习</title>
<script type="text/javascript">
onload = function () {
var txt = document.getElementById("txt");
var txt2 = txt.innerHTML.trim();
// alert(txt.innerHTML.trim().charAt(0));
//alert(txt.innerHTML.trim().substr(0,2));
//函数放在前面！！
var paoMaDeng = function () {
//两个获取字符的方法，charAt(0)和substr(0,1)
var first = txt2.charAt(0);
var rest = txt2.substr(1);
txt2 = rest + first;
txt.innerHTML = txt2;
}
//调用放在后面！！！
aaa = setInterval(paoMaDeng, 100);
}
</script>
</head>
<body>
<p id="txt">
北京欢迎你！</p>
<input type = "button" value = "停止" id = "btn" onclick="clearInterval(aaa);"/>
</body>
</html>