#parentbox {
  display: flow-root;
  position: relative;
  width: 600px;
  border: 1px solid;
  z-index: -2;
}
#box-1 {
  position: absolute;
  top: 25px;
  z-index: -1;

  width: 120px;
  height: 50px;
  margin: 5px;
  background-color: #AAAAAA;
}