you've fought alongside tanjiro before. yet the one thing you seem to question is his hard forehead. how was it so hard that he could knock someone out with just a headbutt?!

"i actually inherited my hard forehead from my mother," tanjiro replied, softly tapping his forehead with his knuckles. he was answering your question. "she saved us from a wild boar by headbutting it with her forehead alone!"