为什么运行不了？
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>jQuery动画特效</title>
<style>
div {
width:100px;
height:100px;
background:purple;
}
</style>
</head>
<body>
<input id="button1" type="button" value="左移"/>
<input id="button2" type="button" value="右移"/>
<div ></div>
<script>
$("#button1").bind("click",function(){
$("div").animate({
left:"+=50px"
},1000)
})
</script>
</body>
</html>
