<script type="text/javascript" src="/js/jquery.js"></script>
<script type="text/javascript">
$(document).ready(function(){ $(".btn1").click(function(){ $("p").text("Hello world!");<p>This is a paragraph.</p>
<p>This is another paragraph.</p>
<button class="btn1">改变所有 p 元素的文本内容</button>