What is most useful when it is used up?高分求脑筋急转弯,用英语回答

What is most useful when it is used up?高分求脑筋急转弯,用英语回答 脑筋急转弯:what is full when it's used and empty when it's at res 脑筋急转弯：1 What's black when it is clean and white it is dirty? 脑筋急转弯,用英语回答:1.What is that ,over the head and under the hat? 英语脑筋急转弯1.What season is the most dangerous one?2.What has ha 英语脑筋急转弯 What belongs only to you,but is used more by others 脑筋急转弯What season is the most dangerous one? 脑筋急转弯（英语）1.What letter(字母）is “you”?It's______.2.What letter 脑筋急转弯“what is the longest word in the world?"请用一个英语单词回答 英语脑筋急转弯what is in the middle of water 英语脑筋急转弯：What part of London is in Fance? 脑筋急转弯 (用英语回答)