A gauge that fits between two gray lines slides all the way along: they’re straight and parallel.
Straight, bent or tilted?
Café wall illusion
Are the gray lines between the rows of tiles straight?
Yes: they are straight and parallel, though they look like tilted wedges.
Why? Where a dark tile meets a light one across the thin gray line, nerve cells in your eye mix up the edges, and the line seems to tilt. In the 1970s, someone in the scientist Richard Gregory’s lab noticed it in the tiles of a café on St Michael’s Hill in Bristol, England, and Gregory studied it.
Picture: Fibonacci, CC BY-SA 3.0 · Wikipedia(opens in a new tab)