The checker shadow illusion is a famous optical illusion that's been intriguing scientists for years, making us perceive two identical shades of grey differently due to the surrounding context.
In this example, we presume that square B will be lighter than A because we know what chequerboards and chessboards look like, and we see that B is lighter than the squares immediately beside it vertically and horizontally.
When I tried to isolate the two individual squares by covering the rest of the composition, square B still looks lighter than square A to me. But a quick check with the eye dropper tool proved that the two squares are indeed identical in hue.
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