3 of our transformations are called isometries. These are transformations that preserve the size and shape of the original figure; they just change the position or orientation of it. Translations, reflections and rotations are all isometries. We know that isometries preserve congruence, since they maintain the size and shape of the original figure.
dilation changes the size of a figure. Since the new figure will not be the same size as the original, they will not be congruent.
