My favorite not-for-the-kiddies story of all-time is Mommy Misplaced by Rachael Ross. In it a mother and daughter swap bodies ala Freaky Friday, but with more interesting results. Unlike the movie, Rachael didn't bother with a fortune cookie mishap. In fact, she didn't bother to give any reason for the plot of her story. It just happens, the way things sometimes do in life, and her characters deal with it.
Now, I for one tend to blame unfortunate (or at least unexplained) events on the government. I read Mommy Misplaced and just knew that some shadowy government agency was behind it. (Other people blame Aliens, the North Koreans, or Miley Cyrus) So I did some digging and discovered the truth. That's what my story is about. Other than using Rachael's story as a springboard for my imagination, they have nothing in common. I'm not trying to write some sort of sequel here, but I wanted to make sure people know who planted the seed. I suppose this could be called fan fiction, now that I think about it.
If I had a nickle for every idea that other writers have inspired in my faulty brain...