body {
    background-color: lightblue;
}


h1 {
    font-size: 50px;
    text-align: center;
}


.container {
    width: 600px;
    height: 350px;
    border: 5px solid black;
    margin: auto;

    display: flex;
    flex-direction: column; 
}


.box {
    height: 100px; 
    background-color: coral; 
    margin: 5px;

    display: flex;
    justify-content: center;
    align-items: center;

    font-size: 20px;
    font-family: Arial, sans-serif;
}
.flex
   {
      /* basic styling */
      height: 350px;
      width: 600px;
      border: 5px solid #555;
      font: 14px Arial;
      margin: auto;
      text-align: center;
      

      
      display: -webkit-flex;
      -webkit-flex-direction: column;

      display: flex;
      flex-direction: column;
   }

   .flex > div
   {
      -webkit-flex: 1 1 auto;
      flex: 1 1 auto;

      height: 30px; 

      -webkit-transition: height 0.7s ease-out;
      transition: height 0.7s ease-out;
   }

   /* colors */
   .flex > div:nth-child(1){ background : red; }
   .flex > div:nth-child(2){ background : yellow; }
   .flex > div:nth-child(3){ background : red; }

   .flex > div:hover
   {
        height: 200px;
   }