Some cages can only be filled one way — spot them and the digits are handed to you.
A cage tells you two things for free: how many cells it has, and what they must add up to. Usually that leaves several possible digit sets — but sometimes it leaves exactly one, and those cells are pinned to that set before you place a single digit. The trick is knowing where to look: for any cage size, only the two LOWEST and the two HIGHEST reachable totals are forced. Two cells: 3 (1+2) and 4 (1+3) at the bottom, 16 (7+9) and 17 (8+9) at the top. Three cells: 6, 7, 23, 24. Four cells: 10, 11, 29, 30. Five cells: 15, 16, 34, 35. Learn to scan for those numbers — it is the first thing an experienced Killer solver does on a new board.
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.