I have some problem with this code, when I click the start button everything is working as expected but when I want to stop animation with `clearInterval` it doesn’t work, just keeps looping... What I am doing wrong? 

```
 var a = function() { 
     $("div").animate({height:300},"slow");
     $("div").animate({width:300},"slow");
     $("div").animate({height:100},"slow");
     $("div").animate({width:100},"slow");
 }

 $(document).ready(function(){
     $("start").click(function(){

         setInterval(a,1000);
     });

     $("stop").click(function(){
         clearInterval(a);

     });

 });

```