浮动的div无法包裹P元素吗？？？？

HTML
<div id="nav"><p>sssssssssssss</p></div>
<div id="content">
<p>fffggggggggggggggggggggggggggggggggggggggggggggggggggggggffgggggggggggggggggggggfffffggggggggggggggggggggggggggfffffffgggggggggggggggggggggggggggggggggffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffgfffffffffffffffddd ddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddfdddddddddddff
</p>
</div>

CSS
#nav {width:300px;float:left;background-color:#FFFF00;border-right:3px solid red;}
#content {width:400px;float:left;background-color:blue;}


content 中我设置了宽度，content的背景色确实显示出了宽度，但是其中的p的内容为什么好像不受限制呢，而且更奇怪的是我在代码中给p的内容加入几个回车，网页中的p的内容还就换行了，这怎么回事，彻底迷糊了。