Teaching a foreigner one's native language is one of those situations where completely arbitrary conventions (including, by the way, how to segment and classify

Teaching a foreigner one's native language is one of those situations where completely arbitrary conventions (including, by the way, how to segment and classify phonemes!) can misleadingly seem like "obvious" facts, and like the foreigner is just being weirdly obtuse by not having a built-in intuition for those arbitrary conventions.