.square, .circle {
    width: 100px;
    height: 100px;
    display: block;
    float: left;
}
.circle {
    border-radius: 50%;
}
#blockRed{
    background-color: red;
    position: relative;
}
#blockYellow{
    background-color: yellow;
    position: relative;
}
#container{
    background-color: blue;
    margin: 0 auto;
    padding: 0px;
    width: 400px;
    height: 400px;

}
#blockWood {
    background-image: url ('https://images.pexels.com/photos/172276/pexels-photo-172276.jpeg');
}