#menu * {
    padding: 0; margin: 0;
}
#menu ul ul {
    position: absolute;
    visibility: hidden;
}
#menu ul li:hover ul {
    visibility: visible; 
}

#menu li {
    list-style: none;
    float: left; 
    position: relative; 
}
#menu li a {
    width: 100px; 
    height: 30px; 
    display: block;
    text-decoration: none;
    background-color: red;
    color: yellow; 
    text-align: center; 
    line-height: 30px; 
}
#menu li a:hover {
    background-color: purple;
}