element_nesting.scss

.parent {
    width: 50%;
    float: left;

    &:before, &:after {
        content: "";
    }
    &:first-child {
        color: red;
    }
    .child-element {
        display: none;
    }
    &:hover {
        color: blue;

        .child-element {
            display: block;
        }
    }
    &.open {
        height: 100%;

        .child-element {
            color: red;
        }
    }
}