Why do girls put their hair across their upper lip?

I have seen innumerous amounts of young females usually from 13-15 pull a lock of their hair in front of their upper lip. Why do they do this? What does it mean? I guess I just don't get it. And I've noticed that its always the immature girls, but that girls who mentality is the same age as an immature 14 year old, but with a 11-12 year old body don't do this. Are they all doing it subconsciously, or are they doing it on purpose?