When I took Formal Logic in undergrad, we had to frequently step up in front of the class and write a derivation or proof on the chalkboards which lined three of the four walls. Our professor called it ‘board work’. One day, after a particularly mid-morning tryst with S5, he stops class, surveys the amalgam of sweet symbols arrayed on the boards before him, and with glee proclaims, “Yes, I think we leave the boards unerased today. It looks impressive enough.”
“Oh! It’s beautiful homework,” she exclaimed. It does seem impressive.
Like my yellow highlight color, we would do board work with yellow chalk. One glorious day, in the same yellow-chalked-equipped building but a different classroom, a history professor of mine walked into class a few minutes late, picked up a piece of chalk, snarled at it, threw it into the wall across the room whereupon it dissolved into sharp, white bows of dust. He swung around and proclaimed, “This is ridiculous. I can’t work with white chalk.” He then stalked out of the classroom. This 30-second whipsaw left us temporarily stunned. He never came back that day. We waited for ten minutes, and then left. I knew he was gone as soon as the chalk exploded into dust.