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Take a piece of paper, crunch it up, put it in a well-lit setting and paint! It's a great exercise to study shapes and relative values. I would suggest a plain paper in order to see value shifts more clearly, but it's up to you! I look forward to seeing all that beautiful crinkled, balled-up paper!
Please be sure to follow the spirit and the letter of the challenge.
For example, each submission must be painted after the challenge is posted.