Lesson objective: Understand that shapes can be partitioned into three equal parts called thirds.

Students bring prior knowledge of partitioning shapes into halves and fourths from 1.G.A.3. This prior knowledge is extended to include thirds as students work to discover if shapes are partitioned equally. A conceptual challenge students may encounter is partitioning circles into thirds.

The concept is developed through work with pictures of plane shapes, which allows students to discover if shapes are partitioned equally by folding and cutting.

This work helps students deepen their understanding of equivalence, because it develops the ideas that (1) shapes can be partitioned into equal parts, and (2) when these parts are put together, the result is equivalent to the whole shape.

Students engage in Mathematical Practice 2 (Reason abstractly and quantitatively) as they use their knowledge of equal parts to understand partitioning shapes in a new way: thirds. Additionally, students continue to think about how the whole is made of a certain number of equal parts (for example, three thirds or four fourths).

**Key vocabulary:**

- fourths
- halves
- partition
- thirds

**Special materials needed:**