I took a class in college where the teacher wrote a long equation on the chalkboard, stepped back, and then quickly erased it, saying that is the last bit of math that we would see. The difficulty with supply/demand curves for people to grasp is that they are one of the best examples...
I love how this building is used today by students and equipped with chalkboards, erasers, desks designed in ethnic patterns, and flat screen TVs. As it was a school day, some of the rooms were in use, but we were able to drop into to several of them: the Czechoslovak, German, ...