Expert technique

Test a Bubble: Trial and Error Without Blind Guessing

Also called trial and error, bifurcation, or proof by contradiction. In Bubble Sudoku it is called Test a Bubble.

When nothing else works, pick a cell with two colors and test one of them. Follow what it forces. If the board breaks, the other color is the answer.

!Before you try this

Turn off Auto Check Mistakes in Settings before you try this on a real board. A test bubble is meant to be wrong, and with that setting on, the game flags a wrong bubble the moment it lands and costs you a life.

When nothing else works

BEFORE

No cell shows a single color, and no color has only one place left, so nothing can be placed. Eight cells show just two colors. The shaded cell shows Red and Light Purple.

Try one of the two

Place a test Red in that cell. You are not keeping it. You are checking whether it breaks the board.

Follow what it forces

The test Red removes Red from the cells it can see. The cell just below and to the right had only Red and Light Purple. Now it has to be Light Purple.

And what that forces

That Light Purple removes Light Purple from its row. Further along the row, a cell with only Red and Light Purple now has to be Red.

The board breaks

Look at the dark shaded row, second from the top. Red could only go in two of its cells. The test Red took one of those columns, and the forced Red took the other. This row has no place left for Red, so the board is broken.

Place the proved color

AFTER

Red broke the board, so Red cannot go in the test cell. Remove the three test bubbles and place Light Purple there instead.

Key cellColor tints match the color being followed

Key Insight

A test is not a guess. You only keep what it proves. If the board breaks, you have proved the other color. If it does not, remove your test bubbles and try something else. Pick a cell with two colors, next to other cells with two colors, and keep the chain short.

Tips and Tricks

Test the color you think is wrong. A chain that breaks proves something; a chain that runs on forever proves nothing.

Cells holding the same two colors as your test cell are where the chain will go next, so look there first.

Keep the chain short. If you cannot hold the moves in your head, you will not trust the answer at the end of it.

The in-game hint runs this whole demonstration for you when the board reaches a point where nothing simpler works.

If you’re stuck on a puzzle, see what to try next, in order.