The middle edge covered with the same gray: one even gray.
Lighter or darker?
Cornsweet illusion
Which is lighter, the gray at the far left or the gray at the far right?
Neither: they are the same gray, though the far left looks lighter. Only a strip by the middle edge is different, fading lighter on its left side and darker on its right. Cover that strip with your finger to check.
Why? Your brain pays most attention to edges, then fills in the areas between them. A light-to-dark edge makes it fill the whole area as lighter or darker.
Picture: Lord Belbury, CC BY-SA 4.0 · Wikipedia(opens in a new tab)