body{
  background: gray;
  font-family: verdana;
}

.wrapper{
  width: 100%;
  max-width: 1200px;
  margin: 40px auto;
}

h1{
  color: #fff;
  font-weight: normal;
  text-align: center;
  font-size: 48px;
}

.panels{
  list-style-type: none;
  padding: 0;
}

.panels li{
  position: relative;
  width: 25%;
  margin: 0;
  display: block;
  box-sizing: border-box;
  float: left
}

.panels li div{
  width: 100%;
  padding: 50% 0;
  text-align: center;
  color: #fff;
  font-size: 20px;
  text-transform: uppercase;
  letter-spacing: 2px;
  line-height: 0;
  cursor: pointer;
}

/*// ANIMATION STYLES //*/

.panels div.back{
  transform: rotateY(90deg);
}

.panels div.front{
  position: absolute;
}

.panels li:hover div.front{
  animation: twirl 0.2s ease-in forwards;
}

.panels li:hover div.back{
  animation: twirl 0.2s  ease-in forwards reverse;
}


@keyframes twirl{
  0%{transform: rotateY(0deg);}
  100%{transform: rotateY(90deg);}
}