Sudoku

Sudoku is the perfect puzzle because it needs no language and no prior knowledge. Nine rows, nine columns, nine blocks, the numbers 1 to 9, and the simple rule that every number appears exactly once in each row, each column and each 3x3 block. From a handful of given numbers you deduce the rest - pure logic, no luck.

Every puzzle here is freshly generated and guaranteed to have exactly one solution. The generator first builds a fully solved grid, then removes as many numbers as the chosen difficulty allows without losing uniqueness. So you are never handed an unsolvable or ambiguous puzzle.

Three difficulty levels decide how many numbers are given and how much you have to work out yourself. Easy usually yields to simple elimination. Medium already demands more combination. Hard forces advanced techniques where you keep several cells in mind at once.

So you do not have to juggle in your head, there are pencil marks. In note mode you jot small candidates into a cell - every number still possible there. That way you work forward systematically, cross out options and find the one number that remains. An eraser removes entries and undo takes back a wrong move.

The check function shows whether your placed numbers match the solution and flags errors before they spread through the whole grid. The timer runs along so you can compete with yourself if you like. And when you truly get stuck, a hint gets you moving again.

Everything runs entirely in your browser. No signup, no data transfer, works offline. The puzzle is generated locally on your device, your entries stay with you. On a phone you tap the numbers in, on a desktop you use the keyboard - both run smoothly.