什么东西当它脏的时候是白的颜色，当它干净时是黑色？

What is whellote when it's dirty and black when it's clean?

youdao

什么东西当它脏的时候是白的颜色，当它干净时是黑色？

What is whellote when it's dirty and black when it's clean?

youdao