What's full when it's used and empty when it's at rest?

What's full when it's used and empty when it's at rest? 脑筋急转弯:what is full when it's used and empty when it's at res What is always filled when it is used and empty when at rest When it's late at 解答英语脑筋急转弯1What's black when it's clean and white when it's d when it's not when it\'s not When's Women's Day It's() what's for it used It's wrong when it's right. 英语翻译sometime it's hard when rain is pouring on.and i used to --What’s it used for?--It’s used for________.