This was done last semester. Our assignment was to draw a crumpled towel with stripes of different local values and to render only half of it so we could clearly see how much of a difference value makes in giving form to an object.
This was one of my favorite exercises because for me it was a breakthrough moment in the course. Usually I try to stay positive and have faith in the process, but every once in awhile that annoying little demon voice of doubt decides to speak up and say something like, how are you going to manage that? To which I responded, fold by fold.