Outcome
You can build, explain, and update a cell's candidate set as the grid changes.
Intro copy
When a cell is not ready to solve, record what is still possible.
Candidates turn an uncertain cell into useful information you can compare with the rest of the grid.
Step 1 — Explain
Target an empty cell.
Copy:
A candidate survives only if the same digit is not already present in the cell's row, column, or box.
Step 2 — Guided example
Prompt
Which digits are still possible in this cell?
Hint 1
Remove every digit already used in the row.
Hint 2
Now check the column and the box. Do not restore a digit once any unit blocks it.
Correct feedback
Good. Those are the digits that remain legal in all three units.
Incorrect feedback
One of those digits already appears in a peer of this cell. Check all three units.
Step 3 — Try it
Exercise spec
Second cell with a three-candidate answer. No guided elimination order.
After completion, place one solved digit elsewhere and ask learner to update the target set.
Correct feedback
Right. Candidates describe the current grid, so they change when a peer is solved.
Incorrect feedback
Re-check the new digit's row, column, and box. Any peer can no longer keep that digit as a candidate.
Step 4 — Practice
- four candidate-building cells;
- two candidate-update tasks;
- one “candidate vs guess” conceptual question.
Checkpoint
Present an unsolved cell with no notes.
Prompt:
Build the complete candidate set, then explain which unit blocks one selected wrong digit.
The second step ensures the learner is not merely matching a pattern.
Completion
All candidate sets legal and complete; both update tasks correct.
Next
Pencil Marks — learn how to record and manage candidates efficiently in the actual game interface.
Reference
Read Sudoku Candidates for Naked/Hidden Singles, subsets, and how candidate information develops into advanced techniques.