jquery添加的新节点触发无效?

<!DOCTYPE html><meta charset=utf-8><html><head><style type="text/css">.main{border:1px solid red;min-height:100px;width:500px;}.main .app{cursor:pointer;}</style><script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script></head><body><div class="main"><div class="app">Good_Old</div></div><input type="button" value="TEST"><script>$(document).ready(function(){$(".app").click(function(){    alert(123);})$("input[type=button]").click(function(){$(".main").append('<div class="app">Good_New</div>');})})</script></body></html>测试发现Old正常，新添的New节点不会弹出123高手指点