This lesson is optional because it is an opportunity for extra practice that not all classes may need. In the first activity, students learn Stage 4 of the Write Numbers center, which was first introduced in IM Grade 1. In this new stage, called Skip-Count by 2, 5, and 10, students write numbers as they skip-count by 2, 5, or 10. In the second activity, students choose to continue working on Write Numbers, or choose between three previously introduced centers focused on addition and subtraction within 1,000.