Also a writer. The first time I was given a writing prompt as a timed exercise I was surprised by what I came up with. If I'd sat and thought of what to write I never would have written what I wrote in a five minute unthinking flurry.
This is arguably true for most writers I think. Not all writing types, of course, but for many. I created an iOS app around this very topic if you're interested, it's fittingly called: Prompts.
