jQuery点击弹出+1放大效果前后台都只有一次有效如何实现？

<!doctype html>

<html lang="en">

<head>

    <meta charset="UTF-8">

    <title>jQuery点击弹出+1放大效果</title>

    <style type="text/css">

        body {margin:0; padding:0;font:12px/1.5 arial; color:#3E3E3E;}

        p {margin-bottom:80px;}

        #btn {width:90px;height:36px;border:none;background-color:#069;color:#fff;font-size:14px;font-family:Microsoft YaHei;cursor:pointer;}

    </style>

</head>

<body>

<p> </p>



<p style="text-align:center;"><button class="btn">点击+1</button></p>

<p style="text-align:center;"><button class="btn">点击+1</button></p>

<p style="text-align:center;"><button class="btn">点击+1</button></p>



<script src="http://www.w3school.com.cn/jquery/jquery-1.11.1.min.js"></script>

<script>

    ;(function($) {

        $.extend({

            tipsBox: function(options) {

                options = $.extend({

                    obj: null,  //jq对象，要在那个html标签上显示

                    str: "+1",  //字符串，要显示的内容;也可以传一段html，如: "<b style='font-family:Microsoft YaHei;'>+1</b>"

                    startSize: "12px",  //动画开始的文字大小

                    endSize: "30px",    //动画结束的文字大小

                    interval: 600,  //动画时间间隔

                    color: "red",    //文字颜色

                    callback: function() {}    //回调函数

                }, options);

                $("body").append("<span class='num'>"+ options.str +"</span>");

                var box = $(".num");

                var left = options.obj.offset().left + options.obj.width() / 2;

                var top = options.obj.offset().top - options.obj.height();

                box.css({

                    "position": "absolute",

                    "left": left + "px",

                    "top": top + "px",

                    "z-index": 9999,

                    "font-size": options.startSize,

                    "line-height": options.endSize,

                    "color": options.color

                });

                box.animate({

                    "font-size": options.endSize,

                    "opacity": "0",

                    "top": top - parseInt(options.endSize) + "px"

                }, options.interval , function() {

                    box.remove();

                    options.callback();

                });

            }

        });

    })(jQuery);

</script>



<script>

	$(function() {

		$(".btn").click(function() {

			$.tipsBox({

				obj: $(this),

				str: "+1",

                callback: function() {

                    //alert(5);

                }

			});

		});

	});

</script>



</body>

</html>