Overview:  The teacher shows students the grocery store problem and asks them to begin by considering a simpler problem.  Instead of working with 7 different points on the map, the teacher asks students to consider how they might locate a new grocery store if they started with two existing grocery stores.  The teacher draws a diagram of two points connected by a segment and asks students to spend a few minutes thinking about how they would find other points equidistant to those two points.  The teacher overhears one student make a comment about circles, and the teacher encourages students to use circles to solve the problem.

Prior knowledge:  The teacher attempts to elicit students’ knowledge of a concept from school mathematics, locating points that are equidistant to two given points.  It is not clear from this episode whether students have formally studied in another setting how to find points equidistant to two given points.  Given that this lesson is designed to introduce perpendicular bisectors, one could assume that students have not yet learned that a perpendicular bisector gives points that are equidistant.  However, students seem to have knowledge of what it means to be equidistant, and this is the knowledge the teacher encourages students to use. 

Other points of interest:  In this launch, the teacher encourages students to use circles as a strategy to find points that are equidistant from two given points.  It seems that the teacher wants students to construct circles because making circles is the first step in the construction of a perpendicular bisector of a line segment.  The teacher seems to think that, by prompting students to make circles, he will provoke students to construct the perpendicular bisector.  This direction could potentially be misleading if students interpret the teacher’s direction as a cue that points equidistant from two given points should all lie on a common circle.