My father was an old-school physics teacher before he retired. He had the last blackboard in his school. It was glass, painted with special blackboard paint. There were signs hung both sides warning the cleaners never to touch it, ever. Apparently there had been an incident when they'd cleaned a blackboard with silicone polish and completely ruined it.
I don't speak spanish, but I'll forever remember "No Borrar Por Favor", because it was written on a large sign stuck to every single blackboard in my high school and college.
