The Trapezoid Illusion
What to see and do: The blue trapezoid's top line is bigger then the green trapezoid's top line, right? Wrong! They are actually the same length. Move the blue trapezoid's top line to compare it to the green trapezoid's top line to see they are the same size.
What's happening: The blue trapezoid's top line seems bigger because the blue trapezoid is much bigger than the green one.
Credit: This illusion is very old, and we learned about it at www.eyetricks.com.