After becoming a demon, Nejuko can’t go out in the sunlight. Most of the daytime she stays in a box. Though she never complained, this is extremely boring to her. After all, why a demon (or human) would like to live in such a tiny box alone! That's why her elder brother Tanjiro decides to give her a problem to solve to bring some excitement.
This is a companion discussion topic for the original entry at https://toph.co/p/bored-in-box