In the position module we learned that adding the z-index to an element only has an effect, if the position property with a value different from static was applied to this element.
One exception from this behaviour is flexbox: Applying the z-index to flex-items (so the elements inside of the flex-container) will change the order of these items even if no position property was applied.
You will need the z-index for flex-items in the following assignment, so keep that special behaviour in mind.