This Instructional Activity is a part of the Lesson 2, Unit 3, Grade 7. The purpose of this activity is to build on students' prior experience with circles and help them refine their definition of a circle. First, students sort pictures of round objects based on whether or not they are circular. Then, they compare the size of the circles to begin a discussion of what aspects of a circle can be measured. Lastly, the teacher introduces the terms diameter, center, radius, and circumference, so students can identify these measurements in the pictures of the circular objects.