The Sander's Parallelogram Illusion
What to see and do: Are the AB and BC lines the same length? Yes, in fact they are the same length. Hard to believe, right? If you don't believe it, drag the slanted box away and the illusion will vanish.
What's happening: The lines are equal in length. It's just the AB line is drawn in a bigger box than the BC line.
Credit: Sander's Parallelogram was described by F. Sander in 1926.