1.(lit.) to sleep on brushwood and taste gall (like King Goujian of Yue 勾践[gōu jiàn], in order to recall one's humiliations) (idiom)2.(fig.) to maintain one's resolve for revenge
inner container (e.g. the rice pot inside a rice cooker, the vacuum bottle inside a thermos, the tank inside a hot water heater, the bladder of a football)