I was being silly, and I was discussing with some friends the wird crackpot theories of one of those guys they call “Pyramidiots” – you know, the sort that “I don’t know how or why to build a pyramid, so the aliens built the pyramids.”
Which, incidentally, it’s a perfect premise for fiction, but utter crap when presented as factual. And the author we were discussing in particular claims that the pyramids were built by Neanderthals.
Yeah.
They were built by Neanderthals before they developed speech.

And so we were talking and being silly, a friend said

they’d need to be great mimes to coordinate the works

and I replied

Yeah, they were led by Marcel Lescaux

And now I know I have to write a story about Marcel Lescaux, the Greatest Mime of the Neolithic.

Because that’s how ideas are born, and it goes to prove that anything, but really anything can be fodder for fiction.
Being silly might not be enough, but it certainly Helps.