清除浮動不成功

我使用同時設置寬度+overflow：hidden；的方法清除浮動并沒有清除成功啊，還是在一行顯示

<!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>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>無標題文檔</title>
<style type="text/css">
.box1{height:50px; background:red; float:left;}
.box2{height:50px; background:blue; float:left;}
p{ width:100%; overflow:hidden;}
</style>
</head>

<body>
?? ?<div class="box1">box1</div>
?? ?<div class="box2">box2</div>
?? ?<p>fndsanfkasjdnfkjafjhffgsdfgsdfgfdaksbjfsdbjfaks</p>
</body>
</html>