I mentioned this story idea in a comment and both gabrielleabelle and penny_lane_42 insisted I write it. So...
100 words, Spike/Buffy, Rating 12.
Feel free to adopt this plotbunny and give it a new home. We could make it a ficathon!
Buffy kills something with a kitchen implement and has sex with Spike afterwards
"A colander, Slayer? Really?"
"What? You don't like my technique?"
"No, actually, I was thinking of making a Caesar salad for lunch and now I'll have to wash the blood off first."
"Wash the blood...? Okay, who killed vampire Spike and put a human doppelganger in his place? Ooh, it's like that time with Willow except in reverse!"
"You think I'm not real, eh? I'll show you who's real!"
"And just how do you expect to--aah! Oh! Mmmm!"
"So, am I real?"
"Mmm, dunno. You could still be a fake Spike. I'll have to do some more checking..."