户 ㄏㄨ ˋ hù 枢 ㄕㄨ shū 不 ㄅㄨ ˋ bù 蠹 ㄉㄨ ˋ dù 戶 ㄏㄨ ˋ hù 樞 ㄕㄨ shū 不 ㄅㄨ ˋ bù 蠹 ㄉㄨ ˋ dù lit. a door hinge never becomes worm-eaten constant activity prevents decay (idiom)