A dilation has a center and a scale factor. It sends every point along the ray from the center, multiplying its distance by the factor. A line through the center stays where it is, a line not through the center goes to a parallel line, and every length is multiplied by the factor, so angles do not change. Two figures are similar when a sequence of rigid motions and dilations carries one onto the other. Similar triangles have equal corresponding angles and proportional corresponding sides, and two equal angles (AA) are enough to prove two triangles similar. This week the crew builds the cardboard scale model and throws shadows with a flashlight.