I also saw this gum ball machine art project at my cousin’s house. The 3rd grade teacher had each kid trace a CD a third of the way down their paper. This served as the glass ball that holds the gum balls. Then they talked about how to show overlapping things (like the gum balls) in artwork and proceeded to make the gum balls. Some kids traced quarters or counting markers (the red and yellow ones used for math), but my cousin didn’t want to. Instead he free-handed it. 🙂  They used pastels to color in the machine and the gum balls, and used watercolor to fill in the background. They also added a shadow to the machine.
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