根由

根由	gēn yóu	the whys and the wherefores / the detailed story / root cause
根	gēn	root / basis / classifier for long slender objects, e.g. cigarettes, guitar strings / CL:条 / radical (chemistry)
由	yóu	to follow / from / because of; due to / by; via; through / (before a noun and a verb) it is for ... to ...