.square {
    box-sizing: border-box;
    border: solid 2px #000;
    border-radius: 30px;
    width: 100px;
    height: 100px;
    overflow: hidden;
    position: relative;
}

.square > div {
    width: 100%;
    height: 200%;
    position: absolute;
    top: 0;
    transition: 400ms;
}

.square > div > div {
    height: 50%;
}

.square > div > div:first-child {
    background-color: rgb(43, 36, 36);
}

.flip > div {
    top: -100%;
}

.col-0 {
    background-color: red;
}

.col-1 {
    background-color: orange;
}

.col-2 {
    background-color: yellow;
}

.col-3 {
    background-color: lime;
}

.col-4 {
    background-color: aqua;
}

.col-5 {
    background-color: blue;
}

.col-6 {
    background-color: blueviolet;
}

.col-7 {
    background-color: fuchsia;
}

.col-8 {
    background-color: maroon;
}

#game {
    width: 400px;
    height: 400px;
    display: flex;
    flex-wrap: wrap;
    flex-direction: row;
    border: solid 2px #ddd;
    background-color: #eee;
}