Which horizontal line is longer than the other?
In this famous illusion, devised in 1913 by the Italian psychologist Mario Ponzo, the upper horizontal line is perceived to be longer than the lower one, even though both are actually the same length. For the past 100 years, we've largely believed that this occurs because the brain interprets the empty space on either side of the lower line as meaning that it's shorter. But recent theories of how the brain works say the perception that the lines are different in length comes from experience. In the real world, lines that converge at the top are generally parallel but are receding into the distance. Railroad tracks, roads, and skyscrapers (seen from street level) all look like this. This view is so commonplace that your brain has become accustomed to transforming such converging lines into parallels.
If you turn the figure upside down, that illusion disappears, because in reality, you almost never see lines that converge toward one another at the bottom and certainly not parallel lines that recede into the distance.