CSS subclasses, am I doing it right?

Question

.popUp
{
    width: 300px;
    height: 300px;
    position: fixed;
    top: 0px;
    left: 0px;
    background-color: white;
}
.question .popUp
{
    width: 300px;
    height: 20px;
    border: 1px solid black;
    background-color: black;
}

As you can see I'm trying to make the question class a subclass of the popUp class. From what I googled this should work, but it doesn't. If I do this instead:

.question
{
    width: 300px;
    height: 20px;
    border: 1px solid black;
    background-color: black;
}

It works, but that misses the point.

Answer 1

popup class is parent container then you should select it as below.

.popUp .question 

so syntax would be parent space child