Prime Sweep

A grid of sixteen numbers. Some are prime; the rest are chosen to look like it. Take every prime — but one wrong pick ends the round, so knowing when you are unsure is worth as much as knowing the answer.

It runs entirely on your device: 0 network requests, works offline. No ads, no account, nothing to buy, and no betting, money or prizes of any kind.

1. Focus the board — click it, or press Tab until it is focused.
2. Move with the arrow keys, take with Enter. Or click a number directly.
3. Take every prime. Taken numbers are ticked.
4. One wrong pick ends the round — so if you cannot rule out a factor, leave it.
5. When the round ends, every number is revealed with its real status.

How do you quickly tell whether a two-digit number is prime?

Rule out the easy factors first: even numbers, anything ending in five, and anything whose digits sum to a multiple of three. What is left needs checking against seven, eleven and thirteen, and that is where people slip — 91 looks prime and is 7 × 13, 119 is 7 × 17, 143 is 11 × 13. You only need to test primes up to the square root, so for a two-digit number that is 2, 3, 5 and 7.

Why do the wrong answers all look prime?

Because a grid full of even numbers would be a scanning exercise rather than a judgement one. The composites here are deliberately odd, not divisible by three or five, and built from factors in the sevens and elevens — the exact family that catches people out.

Why does one mistake end the round?

To make skipping worth something. If a wrong pick only cost points, the best strategy would be to take everything and absorb the loss, which measures nothing. Ending the round means the correct move on a number you cannot resolve is to leave it alone — which is the actual skill.

Does it work offline?

Yes. Once the page has loaded once, the whole game is on your device. Primality is computed locally; there is no server involved at any point.

Is there any money, betting or in-app purchase in this?

None, and there never will be. No currency, nothing to buy, no lives to refill, no wager of any kind.

Can I play it with a keyboard or a screen reader?

Both. Arrows move, Enter takes, and every cell is labelled with its number and its status. Taken, missed and wrong are marked with a tick, a triangle and a cross as well as colour.

Sixteen numbers under 120, roughly half of them prime. Composites are restricted to odd numbers with no factor of three or five, so the round cannot be passed by eye.

Number Span is the other number game here, though it tests how much you can hold rather than what you can work out.

Bookmark this page (Ctrl+D, or ⌘D on Mac) or install the app — it works offline the next time you need it.