Psychologist Mario Ponzo proposed that people determine the size of an object by taking into account the object's background. This is a Demonstration of Ponzo's illusion in which the top line is perceived to be longer than the bottom line, as a result of their relative placement in the triangle. Observe whether a change in thickness, color, or placement affects the perceived change in length.
 Wikipedia. "Ponzo Illusion." (Jun 8, 2016) en.wikipedia.org/wiki/Ponzo_illusion.