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Time to move to the next level! The Hidden Pairs strategy comes into play when we need more advanced analysis of pencil marks. It is a powerful technique that allows us to uncover digits hidden among a tangle of other candidates.
It's like searching for buried treasure - we need to sift through the sand to find two hidden gems. Let's see how it works in practice in our top-left 3×3 box!
Look at the vertical columns intersecting our first box. In the neighboring areas, we have an exceptional situation - the digits 1 and 2 strongly limit the available space.
Now that we have identified our digits 1 and 2, it means that there are only two possible cells for them in the entire 3×3 box.
Since we know with certainty that the digits 1 and 2 must go into these two cells (because they have no other possible positions in the box), it means that no other candidate can remain in these cells.
Look inside these two cells. All the other small pencil marks have now become completely invalid because of the presence of our hidden pair.
Time to clean up! We permanently remove the candidates 3, 4, 6, 7, and 8 from both cells in the first column.
Both cells are now cleaner, which will make solving the entire first column and the rest of the box much easier!
