$j(function(){

    var $el, leftPos, newWidth,
        $mainNav = $j("#main_nav");
    
    /*
        EXAMPLE TWO
    */
    $mainNav.append("<li id='magic-line'></li>");
    
    var $magicLine = $j("#magic-line");
    
    $magicLine
        .width($j(".header_active").width())
        .height($mainNav.height())
        .css("left", $j(".header_active a").position().left)
        .data("origLeft", $j(".header_active a").position().left)
        .data("origWidth", $magicLine.width());
                
    $j("#main_nav li").find("a").hover(function() {
        $el = $j(this);
        leftPos = $el.position().left;
        newWidth = $el.parent().width();
        $magicLine.stop().animate({
            left: leftPos,
            width: newWidth
        })
    }, function() {
        $magicLine.stop().animate({
            left: $magicLine.data("origLeft"),
            width: $magicLine.data("origWidth")
        });    
    });
});