body {
    background-color: #222;
    margin: 0;
    padding: 0;
    font-family: Arial, sans-serif;
}

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

.flex {
    width: 600px;
    height: 350px;
    border: 5px solid #555;
    margin: auto;
    font: 20px Arial;
    display: flex;
    flex-direction: column;
}

.flex > div {
    flex: 1 1 auto;
    height: 50px;
    transition: height 0.7s ease-out;
    display: flex;
    justify-content: center;
    align-items: center;
    color: white;
    font-size: 22px;
}

.flex > div:nth-child(1){ background: #ff5733; }
.flex > div:nth-child(2){ background: #33c1ff; }
.flex > div:nth-child(3){ background: #75ff33; }

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