Blackboards also stop working eventually. What the means is that repeated writing, erasing, occasionally using water to clean them completely, makes tiny little

Blackboards also stop working eventually. What the means is that repeated writing, erasing, occasionally using water to clean them completely, makes tiny little holes in the paint. Then when you write, the chalk gets into the holes and it doesn't erase well with a standard eraser. So you have to use more water, which causes more damage... Blackboards in universities are often painted over with new paint after a few years. Of course, this is relatively simple maintenance.