$(document).ready(function(){

    //When mouse rolls over
    $("#main_nav li").mouseover(function(){
        $(this).stop().animate({height:'150px'},{queue:false, duration:1000, easing: 'easeOutBounce'})
    });

    //When mouse is removed
    $("#main_nav li").mouseout(function(){
        $(this).stop().animate({height:'50px'},{queue:false, duration:1000, easing: 'easeOutBounce'})
    });

});