Back when I first started writing The Idiot Plot for 2002's National Novel Writing Month, the thought that got me going on my story was this: why do some guys going to a public bathroom flush the toilet first, then use it, then flush it again?
I've never found an answer to that: of course, it'd be awk-ward to ask anyone while they're in the bathroom (though I'm nuts enough to have considered asking); but really, why? The only idea I've come up with (other than the novel-world explanation I concocted, and no I'm not sharing that reason yet) is to make sure the toilet's not blocked or something, but seriously, wouldn't the signs of a blocked toilet already be obvious? And my water-saving self cringes at the extra water being used for no clear reason.
(I also wonder if this happens in women's restrooms, but, um, I haven't been in one of those since I was small enough to be carried.)
So that thought helped me get started on The Idiot Plot (15,000 words written in November '02, currently standing at 42,000 words). Something just as random might help me start the (still-untitled) NaNoWriMo '07 masterpiece waiting to ooze out of me...