Draw train tracks around two equal lines, and one looks longer
In the Ponzo illusion, two bars of exactly equal length sit across a pair of lines converging toward a vanishing point, like railway tracks receding into the distance. The bar nearer the vanishing point looks noticeably longer, because the brain reads the converging lines as depth perspective and rescales the 'farther' object as if it must be bigger to look the same size up close. Mario Ponzo first described the effect in 1911.