why `slideDown()` and `slideUp()` does not work true on my menu?  

They are repeated several times with `mouseover()`.
see you: [jsfiddle-my codes](http://jsfiddle.net/5KEnp/)

```
$('#nav li').mouseover(function() {
        $('#nav li ul li').slideDown();
    })
    .mouseout(function(){
        $('#nav li ul li').slideUp();
    })

```