ABSTRACT
Fifteen high school mathematics teachers sat chatting at their monthly after-school professional development seminar while Mark, the seminar leader, passed out five samples of students’ work on the Crossing the River problem. Mark asked the teachers to form small groups to examine the worksheets and identify the algebraic thinking reflected in each solution. While the teachers worked, Mark reminded them to point to evidence from the student work to support their ideas. “Try to start with descriptions of what you see and then use those descriptions to reach an interpretation of the students’ thinking. Then see if you can develop an alternative interpretation.”
