Lesson objective: Understand that the three rigid transformations are rotations, reflections, and translations, and that each one produces an image congruent to the original.

Students bring prior knowledge of congruency as "same size, same shape." This prior knowledge is extended to defining the types of movements that create congruent shapes as students analyze a series of different moves. A conceptual challenge students may encounter is realizing that shapes are congruent, even if they are shown with a different orientation, direction, or position.

The concept is developed through work with game pieces, which allow students to think about transformations in tangible terms.

This work helps students deepen their understanding of equivalence because they will build on to a definition of what it means for shapes to be congruent.

Students engage in Mathematical Practice 4 (Model with Mathematics) as they move from describing physical movements as "turns," "flips," and "slides" to the mathematical vocabulary of rotations, reflections, and translations.

**Key vocabulary:**

- congruent
- reflection
- rigid transformation
- rotation
- translation