Keep Prime Numbers

A storm of numbers hurtles at you — can you click only the primes before the composite swarm buries you?

Numbers start flying at you the second you hit start. Some are prime, most aren't, and your job is simple: click every prime number while letting the composites slide by. Miss a prime and you lose a life; click a composite and the same thing happens. It sounds easy until 51 shows up. Or 57. Or that sneaky 91. The game ramps up quickly. Early waves give you obvious primes like 7, 11, 13 — no problem. Then it throws 27, 33, 49 at you, and your brain has to do factor checks in half a second. I kept losing to 87 before I realized 3×29 = 87. The numbers appear in random spots, so you're constantly scanning the whole screen while your mouse twitches. It's not pure math; it's pattern recognition under pressure. There's no pause to calculate, just tap or don't. What I like is that it doesn't punish you for knowing the Sieve of Eratosthenes — it actually helps. After a few rounds you start recognizing composite 'tells': even numbers (except 2), anything ending in 5 (except 5), and the classic traps like 49, 77, 91, 93. The game feels unfair until you drill those into your head. But once you do, it's a crisp little reflex test. Perfect for a five-minute break, though you might find yourself muttering 'prime, composite, composite' at your screen.

Mouse click/Touch

Drill the prime numbers under 100 into muscle memory before you rely on factor checking. Watch out for 91 — it looks prime but 7×13 will end your run fast. Skip the math when you see even numbers or endings in 5; only 2 and 5 are prime, the rest are traps. Keep your cursor near the center so you aren't dragging across the whole screen. Don't spiral after one wrong click — keep scanning, the next number is already on screen.

Q: Is Keep Prime Numbers free? A: Yes, it's completely free to play in your browser. Q: Do I need to download anything? A: No downloads required. It runs as an HTML5 game online. Q: Can I play Keep Prime Numbers on my phone? A: Yes, touch controls work on mobile browsers. Q: What happens if I click a composite number? A: You lose a life. The game ends when all lives are gone. Q: Is there a time limit for each number? A: Numbers appear and vanish quickly, so you must react fast. Q: Can I pause the game? A: Usually not — pausing would defeat the reflex challenge.