Ms. Chen taught eighth-grade science for twenty-three years. I know this because she was my teacher, and because when I ran into her at a conference ten years after I graduated, she told me she was still teaching — but that the job had become, in her words, "completely different, in the best possible way." --- What she described was a transformation in how class time was used. For most of her career, the dominant activity in Ms. Chen's classroom was explanation. She stood at the front and ex
