In previous lessons, students worked with physical objects and images to determine if groups of objects were even or odd. They represented even numbers as an equation with two equal addends.

In this lesson, students continue to practice identifying even and odd numbers and justifying their identifications. In the process, they deepen their number sense with numbers within 20. Students notice patterns in even and odd numbers when counting and use what they know about 2 equal groups, pairs, and skip-counting by 2 to generalize why adding 1 changes whether a group is even or odd, but adding 2 does not (MP8). Students add even and odd numbers and connect the mental strategies they use to add within 20 to patterns in the value of the sums of even and odd addends.

This lesson is optional because it goes beyond the depth of understanding required to address the standards.

How did students use what they have learned about even and odd numbers to explain the effect of adding 1 or 2? How did students show what they understand about numbers that can be decomposed into 2 equal groups or can be made into pairs? How can you build on that understanding as students consider other equal groups in the next section?