🔢 Sudoku

There is no adding, subtracting or multiplying anywhere in sudoku. The digits are simply nine distinct symbols — swap them for nine colours or the letters A to I and the puzzle is unchanged. What you are actually doing is elimination: working out what must be true in one square given everything already true around it.

That is why people who call themselves bad at maths are often excellent at sudoku. The skill it rewards is patient reasoning, not arithmetic. For what that does to your attention and memory, we went through the research at cerdikcerdik.com/blog/sudoku-brain-benefits.

Scanning is where everyone starts. Pick a digit — say 7 — and find where 7 already sits. Every row and column holding a 7 rules out that whole line for the neighbouring boxes, and two or three placed 7s often leave exactly one square in some box where another 7 can go.

A naked single is a square with only one digit left once you rule out its row, its column and its box. A hidden single is the mirror image: a digit that looks like it could sit in several squares of a box, until you check the crossing lines and find only one of them can take it. Most Easy and Medium boards fall to these three ideas alone.

When you stall, change what you are scanning. If you have been working square by square, switch to working digit by digit — the same board will often give up an answer immediately from the other direction.

Easy boards start with 55 of the 81 squares filled, Medium with 44 and Hard with 26. That gap matters more than it looks. Fewer clues does not only mean more blanks to fill; it means the chains of reasoning joining them get longer, so you must hold more possibilities in your head before anything becomes certain.

The grid flags conflicts as you play — put two 5s in one row and both light up — so Hard is playable without a pencil-and-paper safety net. Undo steps back one move at a time, and the timer feeds a leaderboard kept separately for each difficulty, so an Easy time never competes against a Hard one.

Sudoku is self-correcting: there is a right digit for each square and you find out quickly when you are wrong. That tight feedback loop makes it unusually good as a daily habit. One Easy grid takes most players three to five minutes, roughly the length of a queue at the mamak.

The board sizes itself to your screen width, so a phone gets the largest tap targets it can fit. Tap a square then tap a number; on a keyboard the arrow keys move the selection and the number keys fill it. No account is needed to play — registering only saves your best time per difficulty. When you want a change, cerdikcerdik.com/games has the rest of the collection.