The fragility of certainty

All through my student years I cringed every time an instructor wiped the blackboard with their hands to make a quick correction. Not in the nail-scratching way, but in the "how hard is it to grab an eraser?" way. Well, I was teaching a section last friday, and I finally learned how hard it is to grab that eraser. It was weird, but I found that I couldn't care less if I had chalk on my hands, only the flow of the lecture mattered.