Easy Sudoku

Not the number of clues by itself — that is the common misconception. What makes a puzzle easy is that at every point on the way to the solution there is a cell you can fill in with certainty using only the two simplest techniques:

• A naked single: one empty cell in which only one digit is still possible, because its row, column and box have used the other eight.
• A hidden single: one digit that can only go in one place within a row, column or box, even though that cell could in principle hold others.

Every easy puzzle on this page is checked against that property, not merely given forty clues and hoped for. That is the difference between a difficulty label and a difficulty guarantee.

• Scan for hidden singles box by box. Pick a digit, look at which of the nine boxes still needs it, and see whether the rows and columns already in play leave it only one square.
• Then scan for naked singles. Any cell with eight of the nine digits already among its peers has exactly one answer.
• Repeat. On an easy grid one filled cell almost always creates the next one, and the puzzle unravels in a chain.
• If you get genuinely stuck, use Fill marks. Seeing the candidates written out turns most apparent dead ends into an obvious single.

When you finish easy puzzles without ever having to hunt. Medium is the first level where singles alone will run out and you have to look at how candidates are distributed rather than at individual cells — the point where sudoku stops being a search and starts being deduction.