The reason why the blackboard eraser is not clean

The main reason is that the surface of the blackboard is not smooth, and chalk chips will enter tiny pits when writing or drawing, so it is difficult to clean it with a blackboard eraser with low friction.

When you start using the blackboard eraser, it will be cleaner. But the more you wipe it, the less clean it is because there is a lot of chalk dust on the surface of the blackboard eraser, which acts like a "lubricant" between the blackboard and the blackboard eraser, reducing the friction between the blackboard eraser and the blackboard, thus affecting the wiping effect.