What's a good idea for a short story?

I need to come up with a short story idea. Usually it takes me a while to come up with something, because I am always looking for something creative and unique. I have a short story ignment, it has to be 2-3 pages, double spaced. I want it to be something about a teenage girl's life, something that a lot of girls can relate to. Or I've always wanted to write a mystery short story, something witty and cryptic with a twisty ending (I think they call it a cliffhanger!). Just give me some sort of idea, and I'll develop it from there. Thanks!