I have students do an observational drawing of a paper bag on one side of their paper adding cool colors to shadows and warm colors to highlights.
Then on the back they draw the contours of the bag by holding it up to a window. Then they draw in something unexpected in the bag. Though I usually like to use thick paper with my students, thinner is better for this.
It's important to choose a fairly translucent media for this project. Watercolors, markers, and some colored pencils work well. I assume pastels will also work, but you may want to experiments a bit. We used watercolor pencils for this project. They blend will with just a tiny bit of water on a brush.
When held to the light or displayed on windows, the secret of "what's in the bag" can be seen. This is a great opportunity to introduce students to terms like opaque, transparent, translucent, and the properties of light.
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