I'm trying to wrap my head around the awfully jarring - it hurts my ears so bad, I literally can't bring myself to play it - F natural in m16 of Bartok's "Former Friends" from his Book 1 for Children. The whole piece is in G, has beautiful F#'s all the way until suddenly, out of absolutely nowhere, this F natural pops up and makes my ears bleed.
Is it just me? Or does everybody cringe when they hear it? And if so, why for the love of Apollo did he do this; to torture us? Is there any reasonable answer to be found - except for "ask Bartok!"?
(Also, why are there 2 small vertical lines right before the natural sign?)