The classic 9×9 number puzzle, free in the browser with no account required. Choose from four difficulty levels — easy, medium, hard and expert — and solve the grid using pencil notes you can promote to ink with a tap. When you get stuck, ask for a tip: the game explains the logical technique behind the next move instead of just revealing a number. You can also check a single cell to see if it is correct without spoiling the rest. The app works offline as a PWA and is available on Android.