Here are two vertical lines of identical length. One has reversed arrowheads ('fins') while the other's look normal. One of the lines looks longer!
Something about the arrowheads distorts our perception. It may be our "built-in" sense of perspective.
     Adjust the vertical line on the left by pulling the top, or bottom end.
     When both lines appear to be the same length,
     Press show measure.
     Press reset to do another measure (starting with a randomly selected line length).
    

Credits: Illusion by Franz Müller-Lyer, 1889.

Java Applet: ©1997 Tony Azevedo
http://www.sandlotscience.com/Distortions/Muller_Lyer_Illusion.htm
modified for instuctional usage (changing variables and quantitative measurements),
Eugene Switkes, CfAO/COSMOS Program 2001