重要的思路：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>test2</title>
<style>
.app{
width: 500px;
margin: 100px auto;
background-color: #ccc;
}
.m, .n{
width: 200px;
height: 200px;
float: left;
}
.m{
background-color: pink;
}
.n{
background-color: skyblue;
display: none;
}
</style>
</head>
<body>
<div class="app">
<div class="m"></div>
<div class="n"></div>
</div>
<script type="text/javascript" src="../js/jquery-1.12.1.min.js"></script>
<script>
//核心代码
$(".app").hover(function () {
$(this).children(".n").css("display", "block");
},function () {
$(this).children(".n").css("display", "none");
});
</script>
</body>
</html>
注意：注重核心代码的实现
