Drop-Down Menu with jQuery and css

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

They are 开发者_StackOverflowrepeated several times with mouseover().

see you: jsfiddle-my codes

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You are hiding the ul:

#nav ul {
    display: none;

But you want to slide in the li inside the ul:

$('#nav li ul li').slideDown();

Therefore,

$('#nav li ul').slideDown();

works fine.