Lesson objective: Understand that shapes can be partitioned into two equal shares that we call halves, and that those two shares together equal one whole.

Students bring prior knowledge of modeling and composing shapes from Grade K, Unit 15, K.G.B.5. This prior knowledge is extended to introduce part-whole relationships of shapes as students partition shapes to demonstrate equal shares that we call halves. A conceptual challenge students may encounter is partitioning shapes unequally because they think that any two parts equal halves.

The concept is developed through work with basic shapes, like rectangles, which support an introduction into thinking about fractional parts of a whole (something students will need to know in future grades).

This work helps students deepen their understanding of equivalence because when a whole is partitioned into halves, the halves must be equal parts of the whole. Additional evidence of equivalence in these key concepts is that 2 halves equal 1 whole.

Students engage in Mathematical Practice 3 (construct viable arguments and critique the reasoning of others) as they help a student partition her rectangular room into equal shares.

**Key vocabulary:**

- equal shares
- halves/half
- partition

**Special materials needed:**

- pattern blocks
- shape cut-outs
- tangrams