I remember a prof, I think math, might have been physics (there was one guy in the physics department whose MSc was in mathematical physics, was working on his

I remember a prof, I think math, might have been physics (there was one guy in the physics department whose MSc was in mathematical physics, was working on his PhD, taught in both departments, and seems to my shaky memory to be the best match for what happened) who actually had the eraser in the "other hand" one day, applying it as he went. I think he was just excited, cannot remember why. One or more us said something like "uh, professor...?", he noticed, put down the eraser, and continued, somewhat sheepishly.