I was giving a piano lesson to an elderly woman who had never studied music before, and in an effort to help her understand the concept of one finger per piano key in a 5 finger position, I grabbed a dry erase marker (triple checking that it was such) and wrote the numbers 1, 2, 3, 4, & 5 on my key tops. (I couldn’t quickly find my skinny post-its to write on.) The numbers greatly facilitated the rest of the lesson.
After she left, I attempted to wipe off the marker with a micro fiber cloth I keep handy near the piano. To my dismay, most of the markings did not come off! A moist cloth did not help, either. I picked up the pen I’d used to check the details. Yes, it was a dry-erase marker, yet it had written on it “indelible to porous surfaces.”