五十岁退休做什么:What is clean when it is black and dirty when it is white?

What is clean when it is black and dirty when it is white? What is black when it is clean and white when it is dirty? What is black when it is clean and white when it is dirty? Riddles(What is black when it is clean and white when it is dirty?) What is black when it is clean white when it is dirty? waht is black when it is clean and while when it is dirty? what is important when making phone calls? what is G.I.generation? When I eat,I live,but when I drink,I die.What am I? What is black when it isclean and white when it is dirty?