div#box {
    width: 800px;
    height: 400px;
    border: 1px solid rgba(120, 120, 120, 0.5);
    position: static;
 }
 
 div.ball {
    position: absolute;
    background: black;
    width: 60px;
    height: 60px;
    border-radius: 50%;
    cursor: pointer;
 }
 
 
 input#addBall {
    display: block;
    width: 120px;
    height: 30px;
    margin: 25px auto;
    font-size: 1.2em;
    cursor: pointer;
 }