One of the first things that I have my students work on in their sketchbooks are making tornados. The idea is for the students to move from left to right, darker to lighter, and wider to thinner.
It was really interesting when I first started making these tornados for myself and then watching my students. Most students when they start do really well at pushing hard for the dark and soft for the light. The middle tones are sometimes really hard for students to do.
I have students fill an entire page of tornados. I also have them do some practice before they start filling in their shading for their still life. It makes the shading on the still life better.