.menu {
  display: block;
  margin: 0 auto;
  position: relative;
  width: 200px;
}

.menu > li > a {
  background: blue;
  color: #fff;
  display: block;
  padding: 10px 20px;
  text-align: center;
  text-decoration: none;
  
}

.menu ul {
  background: #ddd;
  height: 0;
  left: 0;
  opacity: 0;
  position: absolute;
  transition: all .5s ease;
  top: 35px;
  width: 100%;
}

.menu li:hover ul {
  height: 200px;
  opacity: 1;
  transform: translateY(0);
}

.menu ul a {
  color: #000;
  display: block;
  padding: 5px 20px;
}