Ladies, what was she wearing?

Now that it's summer, I see the bus full of pretty girls going to work in their short skirts and sandals. It makes me wonder sometimes though, like this morning, there was this girl who was wearing a really short pleated skirt. Like maybe if I was sitting in a seat next to her, I could probably see up her skirt pretty easily.

So when girls wear skirts this short, what is she wearing underneath? Just a thong? How do girls go through the day like that without worrying about presenting her cheeks to men?