When every valid combination of a cage shares a digit, that digit is trapped inside the cage.
Sometimes every single way to fill a cage uses the same digit. That digit is guaranteed to live inside the cage — so if the cage sits within one row, column, or box, that unit’s copy of the digit is used up, and you can erase it from the rest of the unit.
Killer Sudoku keeps every rule of Classic Sudoku — fill the 9×9 grid so each row, column, and 3×3 box holds the digits 1 to 9 exactly once — and adds cages: dashed groups of cells with a small target sum, where no digit may repeat inside a cage. Puzzles usually start with zero given digits, so every deduction begins from the cage sums. That single extra rule unlocks a whole family of arithmetic techniques.
Practise this in Killer mode on Sudoku Challenge, where the in-game Next Step button walks the exact deduction taught in this lesson.
Explore every Classic and Killer technique in the complete Learning Hub, or read how to play Sudoku to get started.