英语趣味问和答 What is black when it is clean and white when it is

英语趣味问和答 What is black when it is clean and white when it is dirty?
What is the poorest in the world?
Why are dogs afraid to sunbathe?
what two words contain thousands of letters?
why is 10*10=1000 like your left eye?
Why is the letter E so important?
what weather do mice and rats fear?
what has three hands but only one face?
What is black when it is clean and white when it is dirty？