It is clean when it is black.It is clean when it is black.It is dirty when it is white.what's it?

问题描述:

It is clean when it is black.
It is clean when it is black.It is dirty when it is white.what's it?

blackboard黑板..

赶紧的时候是黑的,脏的时候是白的,这是什么东东呢?