the red line is a perpendicular bisector of the segment AB. move A or B around and verify that the red line and blue line remain perpendicular. also notice the red lines remains centered between points A and B.

What does it mean for two lines to be "perpendicular" to each other? What does it mean to say "the red line bisects segment AB"? Draw a line that is perpendicular to segment AB, but does not bisect it. Draw a line that bisects segment AB, but is not perpendicular to it.

What path does the midpoint of a ladder trace as bottom of the ladder is pulled away from a wall?