Centers at a Corner
Sketch the area reachable by the midpoint of a line segment of length 1, if its entire length is confined to the first quadrant (x>=0 and y>=0) of a Cartesian plane, and one of its end points must be on the x- or the y- axis.
Number Line Mirrors
Starting with some points on a number line, if we put a mirror somewhere on the number line, it will reflect all existing points in itself so that the resulting set of points are symmetrical around it.