Lesson objective: Understand that perimeter is not additive, i.e., the perimeter of a composite figure formed by smaller polygons is not the sum of their perimeters.

This lesson helps to build procedural skill with calculating perimeter of composite shapes. Rectangles arranged on grid paper are used here because they illustrate why perimeter is not additive. This work develops students' understanding that perimeter is the sum of the lengths of the sides of a polygon.

Students engage in Mathematical Practice 8 (Look for and express regularity in repeated reasoning) as they identify the new side lengths of a composite polygon made of smaller polygons, then calculate perimeter.

**Key vocabulary:**

- perimeter
- polygon
- side

**Special materials needed:**

- grid paper
- 1-6 number cubes
- color pencils