Quest

A row, column or box with one empty cell left: it takes the one digit still missing.

Unlocks Mini 4×4 in free play

Pick a digit a box still needs. Rule out every cell whose row or column already has it: if one cell is left, the digit goes there.

Unlocks Mini 6×6 in free play

The full 9×9 grid, with both ideas so far. A row, column or box with one empty cell left: it takes the one digit still missing. Pick a digit a box still needs. Rule out every cell whose row or column already has it: if one cell is left, the digit goes there.

Pick a digit a row or column still needs. If only one of its empty cells can take it, the digit goes there.

Check which digits a cell's row, column and box already use. If only one digit is left, it goes in that cell.

Unlocks Medium in free play

If a digit's possible cells in a box all lie in one row or column, the digit can't go anywhere else in that row or column. It works the other way too: if a digit's possible cells in a row or column all lie in one box, it can't go anywhere else in that box.

Unlocks Hard in free play

Two cells in a row, column or box that can only hold the same two digits use up those digits, so remove them from the other cells. And if two digits fit only the same two cells, those cells must hold them: remove every other digit from those two cells.

Three cells in a row, column or box that together can only hold three digits use up those digits, so remove them from the other cells. And if three digits fit nowhere else in that unit but those three cells, those cells must hold them: remove every other digit from those three cells.

Unlocks Expert in free play

A digit that fits only two cells in each of two rows, in the same two columns, sits on one diagonal of that rectangle. Remove it from the rest of those two columns. It works the same with columns and rows swapped.

The X-wing idea over three rows: a digit limited to the same three columns in three rows can be removed from the rest of those columns. It works the same with columns and rows swapped.

A cell that can only be A or B sees one cell that can only be A or C and another that can only be B or C. Either way one of them is C, so a cell that sees both can't be C.