I have no idea, but my kid seems to have a basic grasp of it. First up is Black Swan in Australian Roses, drawn in roughly 90 seconds in a moving car on the Garden State Parkway. The ink is a bit more on the red side than I would have liked, but it's interesting.
Next is Apache Sunset, which surprised me. I never thought I would like a yellow/orange ink, but the shading is amazing. I can't believe the different tones that can come from a single ink. Damn, I wish I knew how to draw.