Sometimes, girls wear skirts with their backpacks. This would be fine, if they were aware that with every step they take, their backpack could potentially expose their bum to the world.
I am well aware of this phenomenon, and I only wear skirts which I have tested for no-ride qualities. I used to take pity on these poor girls who didn't have a clue, and would tell them tactfully that their skirt was tucked up under their backpack.
I always tried my best to let them know as soon as possible. In a best-case scenario I would be able to catch up with them before the exposure actually occured. I once RAN up a hill to catch up with a lass whose skirt was perilously close to panty territory. I breathlessly mentioned that her skirt was getting tucked up under her backpack, and all she said was "oh" as she tugged her skirt back down.
What?! No "thank you"... ?AND I RAN!
I watched her skirt get all bunched up again as she walked away. She didn't even care.
I have now become proud and bitter in my old age. Proud at the fact that I NEVER allow my backpack to eat my skirt, and bitter at the fact that these girls made it all the way to college without understanding the basic relationship between gravity and friction.
Skirts with backpacks must go.