When I viewed on mobile phone, I noticed that the three small cards in Red, Green, and Yellow, are not stacking on top of the other. For example, on phone you can see that the Red and Green are side by side and not stacking. How can we change the CSS so that we can make the Green card below the red, and the yellow card bellow the green, when viewed on mobile only?

I tried to stack the cards, using something like this (below), but I am not sure if that's the proper way of doing this.

```
    @media (max-width: px)  {
    .flex-container {
}
}

```