I've taught this using popcorn. My unit is called "Variation of Form" I
give each student a little bit of popcorn and ask them to find an
interesting form. I have them draw the form with values in their
sketchbook. I then ask them to redraw the image to larger paper and
instruct on gesture studies of popcorn. Then I ask them to redraw the
image with a slightly cubist style. Finally we create the popcorn in 3D
using Papier Mache. At the moment I have giant popcorn hanging from my
ceiling in my art room. Of course my students think I'm nuts.
Hope this helps!