.parent {
  display: flex;
  font-family: arial, sans-serif;
 }

.item {
  width: 50%;
  padding: 2em;
  color: #ffffff;
  background: #eeeeee;
  min-height: 500px;
}

.last {
  color: #222;
  background: #ccc;
}

@media screen and (max-width: 600px){

 .parent {
    flex-direction: column-reverse;  
  }

 .item {
    width: auto;
    min-height: 200px;
 }

 .last {
  background: #aa0000;
 }