Grade 8
CCSS Math Standard: 8.G.5
Understand congruence and similarity using physical models, transparencies, or geometry software. Use informal arguments to establish facts about the angle sum and exterior angle of triangles, about the angles created when parallel lines are cut by a transversal, and the angle-angle criterion for similarity of triangles. For example, arrange three copies of the same triangle so that the three angles appear to form a line, and give an argument in terms of transversals why this is so.
A lesson about lines, line segments, and rays that avoids dull memorization. Instead, we ponder this delightful question: Which is longer, a ray or a line? Then, kids consider what these different geometric concepts would think about each other.