Every so often, I browse through e-publisher submission pages, looking at what 'special calls' they have at the moment. I know some authors do that to see what genres are hot or are predicted to soon be all the rage. For me, it's more a way to challenge myself.
When I first started writing in English - 10 years ago, now that I think about it - I applied my words to fanfiction, borrowing characters from a favorite TV show and giving them new adventures. My very first fanfic was an answer to a challenge: someone wanted to see a specific scenario play out, and I was intrigued enough by the idea to write it. Over the years, many of my fanfic stories started as challenges; I really do enjoy writing with someone in mind, following the directions they gave me and wondering if they'll like the result.
So to go back to my browsing of 'special calls' pages...
Once, I was hooked by a call by Samhain for space opera novellas. I'd never written one, but I love the genre and I thought it was a great way to stretch my writing muscles. This was the start of Moonlust.
More recently, I looked at the Torquere page and one word in their announcement tickled my muse: ghost. I'd never written a ghost story. It looked like it might be fun... and it was! In two days, I had close to 9000 words, which was actually longer than the submissions Torquere wanted. I trimmed the story to size... and that's how The Thirteenth Halloween came to be!
I'm always on the look out for challenges, by the way. If you'd like to throw one at me, check out my 'prompt me' page.