Bubble Sudoku is pure sudoku: nine colors instead of nine numbers, and the same rules that have kept the puzzle sharp for decades.
Want worked examples? See the techniques.
The board is a 9×9 grid divided into nine 3×3 blocks. Some bubbles are already placed as clues. Fill every empty cell to solve the puzzle.
Every row must contain each of the nine bubble colors exactly once. No color may appear twice in the same row.
Every column must also contain each of the nine bubble colors exactly once, with no repeats from top to bottom.
Each of the nine 3×3 blocks must contain all nine bubble colors exactly once. Every puzzle is built so these three rules leave exactly one solution.
When a row, column, or block has just one empty cell, only one color can go there. Count the colors already in place and add the one that is missing.
Look at the row, the column, and the block an empty cell belongs to. Every color already placed in them is ruled out, and whatever remains is what the cell can still hold.
Tap Notes to switch into pencil mode. Add a small dot for each color that could still go in a cell, and remove dots as you learn more. The Notes card on the Techniques page shows how to use them well.
Every Bubble Sudoku puzzle has exactly one valid solution reachable by logic alone, so you never need to guess. Hints are there if you get stuck.
