Lesson objective: Reason to differentiate between geometrically defining attributes and non-defining attributes.

This lesson helps to extend understanding of attributes. Students will discuss non-defining attributes such as color, shape, and size of an object. Students will consider the "must haves" or defining attributes of a shape. Students will describe in their own words why a shape belongs to a given category. Two different attributes are used here because they encourage students to engage in problem solving that is about thinking and reasoning. This work develops students' understanding that shapes both defining and non-defining attributes.

Students engage in Mathematical Practice 3 as they construct viable arguments and critique the reasoning of others. Students will use clear definitions with peers as they define attributes and provide a viable argument reasoning why a shape lacks specific attributes. Students also engage in Mathematical Practice 7 as they look for and make use of structure. Students will use the patterns seen in different shapes to make comparisons. Students will use the relationships demonstrated in the defining attributes to explain similarities and differences.

**Key vocabulary:**

- attribute
- defining
- non-defining

**Special materials needed:**

- envelopes, string