Topic C gives students a chance to explore the usefulness of measuring with similar units. The topic opens with Lesson 7, where students measure the same objects from Topic B using two different non-standard length units simultaneously, such as toothpicks and small paper clips (1.MD.2). They then use small paper clips and large paper clips, two non-standard units that happen to be the same object but at different lengths. Each time they measure one object using both units and receive inconsistent measurement results. Students then begin to ask the question, “Why do we measure with same-sized length units?” As they explore why it is so important to use the same-sized length unit, they realize that doing so yields consistent measurement results. In Lesson 8, students explore what happens when they use a different unit of measurement from that of their classmates. As students measure the same objects with different non-standard length units, they realize that in order to have discussions about the lengths of objects together, they must measure with the same units. Students answer the question, “If Bailey uses paper clips and Maya uses toothpicks, and they both measure things in our classroom, will they be able to compare their measurements?” With this new understanding of consistent measurement, Lesson 9 closes the topic, with students solving compare with difference unknown problems using centimeters. Students explore and solve problems such as, “How much longer is the pencil than the marker?” (1.OA.1). Revisiting the centimeter here helps students recognize the value of having a consistent way to communicate about various measurements.