What Are We 3

Riddle: We are two. The first is black when clean, white when dirty; Smooth when new, rough when old; and flat usually on the wall. The second one is white when clean, black when dirty; long when young, short when old; has no legs but moves, usually from left to right. The more we touch each other, the more one become dirty; and the other becomes shorter. We are two of a writer's materials. What are we?

Answer: A Chalkboard (Blackboard) and a piece of white Chalk. Source: https://riddles.com/7329

We are two. The first is black when clean, white when dirty; Smooth when new, rough when old; and flat usually on the wall. The second one is white when clean, black when dirty; long when young, short when old; has no legs but moves, usually from left to right. The more we touch each other, the more one become dirty; and the other becomes shorter. We are two of a writer's materials. What are we? — What Are We 3 by Moroyei Bayelsa Ebiakpo Riddle