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Place one Arnold in every row, every column and every coloured region, and make sure no two Arnolds touch, not even diagonally. Every puzzle has exactly one answer, and you can always reach it by logic alone.

• Tap a cell to cross it out when you know Arnold can’t go there. Tap again to clear it.
• Drag to cross out (or clear) a whole run of cells at once.
• Double-tap a cell to place Arnold. Once he’s placed, he stays.
• Undo takes back your crosses. Restart clears the whole board.

They’re your lives. Each puzzle gives you three. Placing Arnold in the wrong cell costs a treat, and if you run out it’s nap time: you can try the puzzle again straight away. Crossing cells out never costs anything.

A hint points out the next logical step, highlights the cells involved and explains the technique behind it. If it has cells to cross out, it can do that for you, but it never places Arnold.

You start with 3 hints and can hold up to 7. Using one spends one. You earn one back the first time you solve each level and each day’s Daily Challenge. Asking again before you change the board shows the same hint for free.

Solve a puzzle with no mistakes and no hints. A mistake costs one star, and using a hint costs one. You can replay any level you’ve solved from the Levels screen to improve your stars.

Every day at midnight UTC (1am in the UK during British Summer Time). Everyone gets the same puzzle. Solve it on consecutive days to build your streak; missing a whole day resets it.

Only on your device. There are no accounts, so progress doesn’t sync between devices, and deleting the app deletes your progress. A puzzle you leave halfway through is saved, so you can pick up where you left off.

Open Settings (the cog) and choose Wipe all progress. It asks you to confirm first, and it can’t be undone. Your sound settings are kept.

Yes, each has its own switch in Settings. The game also stays quiet when your iPhone’s silent switch is on.

Arnoldoku is £1.99 on the App Store (or the equivalent in your country), paid once. After that there’s nothing more to pay: no ads, no in-app purchases, no subscriptions and no accounts. It plays fully offline, on iPhone and iPad.