A transformation is a function: every point of the plane goes in, and one point comes out. Translations, reflections and rotations are rigid motions, because they keep every distance and every angle. A stretch does not. Each rigid motion has a precise definition using segments, perpendicular lines, parallel lines and circles, and a coordinate rule on the grid. Some figures are carried onto themselves by certain rotations and reflections: that is symmetry. A sequence of motions can carry one figure onto another, which is where congruence begins next week.