I know friends are scary.....but thats part of the college experience.
Sometimes its just that wording one certain questions was unclear such that lots of students misunderstood the question, or new information or a better way of explaining a section (or realizing a section was being glossed over too quickly) is often the motivation. Education, like science, is not static, and sometimes one idea of how to present a concept seems really good if you are a professor who understands it, but lacking for someone who has no clue whats going on.
Note: My experience is from a chemist whose PI wrote an intro chem book for college, might not be the same in other fields.
its 50-50 since 3D printing and bitcoin became old hat.
Air pollution is really making us pay through the nose.