In this third in the sequence of three optional lessons, students examine tessellations using non-regular polygons. Students first study triangles and show that any triangle can tessellate the plane. This work calls back to an important idea that students studied in grade 6: two copies of a triangle can be put together to make a parallelogram. Next, students show that all quadrilaterals can also tessellate the plane by using rigid motions and the fact that the sum of the angles in a quadrilateral is always . Lastly, students return to pentagons. While not all pentagons can tessellate the plane, students investigate a pentagon whose tessellation leads to some rotational thinking. Through experimenting with these shapes, students construct arguments about which polygons can tessellate the plane and why (MP3) and apply their knowledge of interior angles of polygons to support their arguments (MP7).