One explanation for the Müller-Lyer illusion is based on depth perception, suggesting that our brains interpret the 2D figures using 3D spatial cues. The line with outward-pointing fins resembles the outside corner of a building projecting toward the viewer (appearing closer), while the line with inward-pointing fins can be seen as the inside corner of a room receding into the distance (appearing farther away). According to the principle of size constancy, if two objects create retinal images of the same size, the object perceived as being farther away is interpreted as being physically larger. This principle helps explain why the different fin orientations lead to a misperception of line length.