How do you make this function animated?

[Laycee]

How do you make this function animated?

$(function() {
    $(window).scroll(function() {
        if($(window).scrollTop() > 100) {
            $(".header").css({
                width: "100%",
                position: "fixed"
            })
        } else {
            $(".header").css({
                width: "auto",
                position: "relative"
            })
        }
    });
});

[terrea]

$(function() {
    $(window).scroll(function() {
        if($(window).scrollTop() > 100) {
            $(".header").animate({
                width: "100%",
                position: "fixed"
            },5000);
        } else {
            $(".header").animate({
                width: "auto",
                position: "relative"
            },5000);
        }
    });
});