Answer:
What is black when you buy it red when you use it and gray when you throw it away?
What is black when it's clean and white when it's dirty?
To cross the water I'm the way. For water I'm above I touch it not and truth to say I neither swim nor move.
I build up castles. I tear down mountains. I make some men blind, but I help others to see. What am I?
I have lakes but no water forests but no trees and cities but no buildings. What am I?
No matter how little or how much you use me, you change me every month.