The below flash presentation is an example of the Poggendorff illusion.
WikipediaIt can be a very strong illusion at first. Trying to line up the top and bottom lines so they meet can be tricky. Most of us though will not be fooled for long and some may not be fooled at all. I can align the two lines just about every time now. It has reached the point that I doubt if it will be a challenge for anyone. Maybe it never was a strong illusion, sometimes my memory plays tricks on me. This illusion seems to be stronger with kids.
The Poggendorff Illusion is an optical illusion that involves the brain's perception of the interaction between diagonal lines and horizontal and vertical edges. It is named after Johann Poggendorff (1796-1877), a German physicist who first described it in 1860.
Move one of the lines sticking out of the box until you think it matches up with the other one. Now press the button.
Do the lines match up?