About XXI

XXI is one small puzzle a day. Everyone in the world gets the same 4×4 grid of digits, and the job is to draw a path through it that adds up to exactly 21. Anyone can find a short answer. The game is in finding the long one.

Why it exists

I'm Devanshu Chaudhary, an iOS developer who has shipped apps for years under the name Sassy AI. I've wanted to make a game around the number 21 for a long time, and a once-a-day puzzle felt like the right shape for it: something that takes a minute, gives everyone the same challenge, and then leaves you alone until tomorrow. No energy bars, no endless mode, no account.

How every grid is made

Each day's grid is generated from the date by a fixed algorithm, so every player, on the web and in the iOS app, gets exactly the same one. Before a grid is used, a solver checks every possible path through it. A grid only goes out if it has a quick three-tile answer and a long answer of at least seven tiles. If it doesn't, the generator throws it away and tries again.

That check is why every XXI has a "long way" worth hunting for, and why the result card can tell you the best possible score for the day. Once a day is over, the full answer is published in the archive, along with how many people played and how many found the long path.

What's on the site

• Today's puzzle, free and playable in any browser.
• The archive of past grids and their answers.
• Guides on strategy, mental maths and other daily puzzles worth your time.
• The iOS app, with the same daily grid, streaks and a daily reminder.

How it's paid for

The site is free and shows a small number of ads. If you'd rather not see them, you can remove them on the web with a one-time purchase, or separately in the iOS app. That's the whole business model. Your game data stays on your device; see the privacy policy for the details.

Get in touch

Found a bug, have an idea, or want to tell me about your nine-tile path? Email support@averysassy.ai or find me on X at @devdchaudhary. I read everything.