For our second lesson, everyone painted white objects that had colored papers reflected on their shadow sides. The main point of the lesson was that white is almost never pure white, but is easily (and often beautifully) influenced by its surroundings.
My students sometimes have trouble with tinting white and keeping its value light, so this was a good exercise for using restraint when mixing in other hues.
Here is one student example with sensitively observed whites:
The third lesson was focused on painting glass. Each set up included two glass objects, one with nothing but white paper behind it and the other with colored paper and a mango. Everyone had to paint the simpler one first, then tackle the one with the mango.
The idea is that when you paint glass, you aren't really painting the glass. Instead you are painting what you see behind it. If a glass has nothing behind it, it likely is just ever-so-slightly darker in value than the background. You begin by simply filling in the entire shape with this color/value. Some under-reported edges (just the ones you see when you squint) and a couple of selective highlights can complete the feeling of glass.
For the other glass object, the lowered intensity of the colors of the mango and colored paper as well as the distortions as seen through the glass give the vase its transparency.
A student example:
