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Poggendorf's Illusion  This illusion is one example of "angle dilation" illusions for which a physiological explanation has been proposed [see Blakemore, Carpenter, Georgeson, Nature,228,37-39 (1970)]  
This illusion consists of a straight line which disappears behind a solid rectangle and then reappears on the other side.
How are you at lining things up? Test yourself with the "POG." Move the top portion of the line back and forth until it lines up with the bottom segment. All done? Straight and true?
Click the 'show' button. How close were you? Bummer. Well, you're not alone. Click 'hide' to try again, or 'reset' to correctly align the two segments.

Credits: Proposed by Johann Poggendorf in 1860.   
Java Applet:
1997 Tony Azevedo
http://www.sandlotscience.com/Distortions/Poggendorf_Illusion_I.htm
modified for instuctional (changing variables and quantitative measurements), Eugene Switkes, CfAO/COSMOS Program 2001