Hvorfor virker min drop down menu ikke?

<link href="css.css" rel="stylesheet" type="text/css" />

<script type="text/javascript">
menu1synlig=0 //menuen er skjult

function visMenu(objekt){
    if (menu1synlig==1){
        collapse(objekt);
        menu1synlig=0;
        return;
    }
    if (menu1synlig==0){
        expand(objekt);
        menu1synlig=1;
    }
}


  </script>

<div id="topmenu1">
<a class="knap" href="java script:visMenu('menu1');">Solsystemet</a>
<div id="menu1">
    <div><a class="knap" href="blank0.php">Introduktion</a></div>
    <div><a class="knap" href="blank0.php">Indre planeter</a></div>
    <div><a class="knap" href="blank0.php">Asteroiderne</a></div>
    <div><a class="knap" href="blank0.php">Ydre planeter</a></div>
    <div><a class="knap" href="blank0.php">Kometskyen</a></div>
</div>
</div>


css.css
a.knap {
background-color: #900;
color:#fff;
display: block;
font: normal bold 12px arial, helvetica, sans-serif;
text-align: left; text-decoration: none;
margin-bottom:1px; border:2px outset #d00;
padding: 2px 12px 2px 12px;
width:130px; height:22px;

}

html>body .knap {
width:102px; height:14px;

}

a.knap:hover {
background-color: #cc0000;
color: #fff;
text-decoration: none;
border-style: inset;
}

#menu1 {
display:none;
}