I personally love the way learning math is set up on this site. Variable practice during the mastery challenges is best for learning and retaining knowledge over long periods of time, and winning badges is such an encouragement. It's like a game for students, and I love it when learning is fun! I think it would be good to try a similar set up for physics and chemistry. I think it will help students see how much fun the sciences are and really hone their STEM skills.
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