Six elementary school teachers uncover an interesting paradox in the data that they have just collected during a 4th grade mathematics lesson on pattern growth. Most students correctly filled out a table that related the number of tiles in a pattern to the pattern's perimeter, but many students were unable to express this information in words or as an equation. These data suggest that the table “spoon-fed” the students. The teachers—the one teaching the lesson and the five observing it—redesign the lesson, eliminating the worksheet that contained the table. Two days later, another of the six lesson study team members presents the redesigned lesson to a different class of 4th graders while her colleagues once again observe. She discovers that students grasp the pattern as they work at organizing the data themselves instead of just filling in a table that organizes the data for them. One team member reflects on the experience of planning, teaching, observing, revising, and reteaching the lesson: “I learned that a worksheet can be a dangerous thing.”

These teachers are practicing lesson study, a professional development approach that originated in Japan. Educators have credited lesson study with bringing about Japan's evolution of effective mathematics and science teaching (Lewis, 2002a, 2002b; Lewis & Tsuchida, 1997, 1998; National Research Council, 2002; Takahashi, 2000; Yoshida, 1999a, 1999b). In 1999, The Teaching Gap brought Yoshida's account of lesson study to a broad public audience (Stigler & Hiebert, 1999; Yoshida, 1999a, 1999b). Lesson study has subsequently swept across the United States, springing up in at least 250 schools in 29 states (www.tc.columbia.edu/lessonstudy).