Spot the Difference: Two Grids, One Change

Comparing two images is much harder than it feels, because you cannot look at both at once. Your eyes jump between them and your working memory has to hold one side while it inspects the other.

That is why scanning row by row beats staring: it keeps the amount you have to remember down to a single line at a time. Wrong taps cost three seconds, so a careful sweep pays for itself.