Star Wars
I am working on using my pen and thumb to measure perspective. Use your pen or thumb to help sketch a 3-dimensional image onto a 2-dimensional media. Holding your arm against your body, you can see down your pen or thumb to gauge how key landmarks line up together—another skill to learn on my sketching journey.
The simplicity of one pen on a sheet of paper has been great. No decisions or angst over pens, colors, paper vs. sketchbooks; pull out my pen and pad and draw.