.square, .circle {
    width: 100px;
    height: 100px;
    display: block;
}

.circle {
    border-radius: 50%;
}

#blockRed {
    background-color: red;
    position: relative;
}

#blockYellow {
    background-color: yellow;
    position: relative;
}

#container {
    width: 400px;
    background-color: blue;
    margin: 0 auto;
}

#blockWood {
background-image: url('wood.jpeg');
}