How it works
A cell has only one candidate digit left — every other digit 1–9 is already used somewhere in that cell's row, column or 3×3 box. Whatever's left is the only digit that can possibly go there.
Try it
Let's find out why Row 5, Column 6 can only be one digit. Tap every digit that's still possible there — check what's already used in its row, column and box.
Tap every digit that's still possible in Row 5, Column 6 — check what's already used in its row, column and box. Press "Check my work" when you're ready.
Nice — every candidate for Row 5, Column 6 is confirmed.
Not quite — look again at what's already placed in each cell's row, column and box, then check again.
Only one digit survived: 1. Since every other digit 1–9 is already used somewhere in Row 5, Column 6's row, column or box, 1 is the only one left.
Since 1 is the only candidate left, tap Row 5, Column 6 and place it.
That's it — nicely done.
How to pass this stage
The tour above is practice — it doesn't submit anything and doesn't pass this stage by itself.
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1
Beat a puzzle. Play one of the puzzles below all the way through — or any puzzle you've already solved; it doesn't have to be a drill.
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2
Open its trophy room. A trophy icon appears at the top of that play's own page once you've beaten it, never before. Tap it to open the trophy room.
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3
Name Naked Single. Pick the cell where you spotted it, choose Naked Single and the digit, and submit. Get it right and this stage passes right away, unlocking the next one; get it wrong and it costs one of three strikes on that puzzle, but never blocks a retry on another.
Drill it
Real puzzles that genuinely contain this technique somewhere in their solve — any of these will work for the steps above.