.invader, .help-box {
    width: 64px;
    height: 32px;
    background: url(../img/spaceinvaders.png);
    background-size: cover;
    position: fixed;
    top: 200px;
    left: 50%;
    transform: translateX(16px);
}
.anim-alien-die {
    animation: explode 2s forwards;
}

@keyframes explode {
    0% {
        opacity: 1;
        transform: translateY(0px);
        filter: contrast(-175%) brightness(100%);
    }
    5% {
        opacity: .8;
        filter: contrast(50%) brightness(1000%);
        box-shadow: 0px 0px 30px 15px #fff;
        background-color: rgba(255,255,255,.75);
        background-size: 100%;
        background-position: center;
        background-repeat: no-repeat;
        height: 64px;
        width: 64px;
        border-radius: 50%;
    }
    10% {
        transform: translateY(0px);
    }
    100% {
        opacity: 0;
        transform: translateY(300px);
        box-shadow: 0px 0px 4px 2px #fff;
        background-color: rgba(255,255,255,.25);
        background-size: 100%;
        background-position: center;
        background-repeat: no-repeat;
        height: 64px;
        width: 64px;
        border-radius: 50%;
    }
}