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 numbers one at a time. A removal only stays if a solver still finds exactly one solution. So you are never handed an unsolvable or ambiguous puzzle.

Three difficulty levels decide how many numbers are given and which deductions you need. Easy has 38 givens, and at every step only one digit fits some cell. Medium has 31 givens and sometimes makes you look for the only place of a digit. Hard has about 25 givens and sometimes needs advanced tricks such as pairs and block lines. The line above the puzzle says what it really asks for.

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 numbers clash in a row, column or block and marks those cells. Every completely and validly filled grid counts as solved. The timer runs along and pauses when you press pause or leave the window. And when you truly get stuck, "Show solution" reveals everything, but then it does not count as solved.

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.