Sudoku

How to play sudoku

1. Press PLAY to enter Sudoku Springs and open your first grid.
2. Tap an empty cell to select it, then tap a digit to place it.
3. Selecting a cell dims its row, column and box, and lights up every other cell holding the same digit.
4. Tap the pencil to switch to notes, and tap digits to jot candidates into a cell.
5. Placing a digit clears that candidate from every cell it can no longer go in.
6. A digit that repeats one already in its row, column or box is refused, the cells that clash light up, and it costs a life.
7. Any other digit goes on the board, and the undo key takes back whatever you put down.
8. Stuck? The first hint is free: it names the technique that cracks the board from here and lights up where to use it.

A hint that teaches instead of answering

Every sudoku app answers "I am stuck" the same way: it fills a cell in. You learn where a digit went and nothing about how it was found, which is why the most popular request from people who love these games is a hint that explains itself. Here the hint is a ladder and you decide how far down it to go. The first rung names the technique and the row, column or box to use it in, and it costs nothing. The second explains the reasoning in the words solvers actually use, like a hidden single or a pointing pair. The third points at the cell, and only the fourth gives you the digit. Walking the whole ladder costs exactly what a plain reveal costs, so learning is never the expensive option. The advice comes from the same solver that graded the board, so a grid can never promise a difficulty its hint cannot explain.

The board judges your digits, not a stored answer

Most sudoku apps compare every digit you place against the finished grid they are holding and take a life whenever it differs. On the board in front of you the move was perfectly legal, so you get punished for not guessing the answer. Here the only thing that counts as a mistake is one you could see: a digit that repeats another in the same row, column or box. That one is refused, and the cells that prove it are lit up so you can check. Everything else goes down exactly as it would in a newspaper, and if it turns out to be wrong you find out the same way you would on paper, when some cell runs out of digits and the board says so.

Every grid has exactly one solution

That sounds obvious and it is not: a sudoku with two valid solutions lets you fill a perfectly legal grid and still be told you are wrong, with nothing you could have done differently. Every board here is checked for uniqueness as it is built, on each clue removed, and then checked again from scratch before it ships.

Difficulty is a promise, not a guess

It is measured by TECHNIQUE, not by how many digits you start with. A grid with 26 clues that falls to plain scanning is easier than one with 30 that does not, so counting clues tells you almost nothing. Each board is built by digging clues away only while it stays solvable by the techniques its level allows, and it is rejected if it never ends up needing the one its level promises. So a grid marked as needing pencil marks really does need them, and an easy grid really never will.

• Easy Scan the boxes. No pencil marks needed.
• Medium Pencil marks start to pay off.
• Hard Needs a pair or a locked digit.
• Expert Goes past the standard techniques. Take your time.

You pick the one you want from the lobby, and it applies to your next grid. Grids are built as you play rather than handed out from a fixed pack, so a level never runs out and you never have to climb through easy ones to reach a hard one. Each level also has a page of its own: easy sudoku, medium sudoku, hard sudoku, expert sudoku.

The daily grid, and learning the techniques

There is a daily sudoku that everybody gets on the same day, with a leaderboard that ranks how cleanly you solved it rather than how fast. If the vocabulary is new, how to play covers the rules and the first two moves, and there is a page for each solving technique with a worked example our own solver produced and checked, from the hidden single to the naked triple.

More free puzzles at BrainPuzzles

In the mood for something else? Hunt hidden words in our word search puzzles, unscramble a letter wheel, read a trail of brain riddles, clear a board of mahjong tiles, or compare two pictures in spot the difference. Every game saves your stars and streak as you go.