JQuery .slideDown() 向上滑动

This works but I'm not sure why. In function capIn(), in my mind the line $botcap.slideDown("slow") should slide the div down. It slides it up. If I try using .slideUp() nothing happens as if it is trying to slide it down. Can anyone explain this to me?

$(".slide").hover(capIn, capOut);

function capIn(){
    //slide top caption down
    var $topcap = $(this).children(".topcap");
    $topcap.slideDown("slow");

    //slide bottom caption up
    //!! Why does slideDown slide caption up here?
    var $botcap = $(this).children(".botcap");
    $botcap.slideDown("slow")
}

function capOut(){
    //slide top back up
    var $topcap = $(this).children(".topcap");
    $topcap.slideUp("slow");

    //slide bottom back down
    var $botcap = $(this).children(".botcap");
    $botcap.slideUp("slow");
}

解决方案

jQuery's slideDown and slideUp functions are actually misnomers. As the documentation for slideUp puts it:

Hide the matched elements with a sliding motion.

The hiding is achieved by modifying the height of the element; normally, this means that the lower edge of the element appears to slide up, hence the name. However, if the element is anchored at the bottom (e.g. by setting position: absolute and bottom: 0), the height modification will make the top edge appear to slide down.