One move changes only the conditions that pass through that cell — here, the blue-framed row, the pink-framed column and the green window. Distant clues have not changed, so there is nothing to recheck. Looking only where things changed is called locality, and computers use the very same trick to stay fast. Rescanning the whole board every move is diligent — and wasteful.

In the app, one new tip arrives for each day you play.

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