Primes: Prime or Composite

A number from 2 to 999. Is it prime, or composite? Primes is a small, quiet game for spotting prime numbers by hand. Work it out in your head, tap Prime or Composite, and see exactly how the answer is reached: the square-root bound, the divisibility trick that cracks it, the factors it splits into. A miss still teaches you something. WHAT'S INSIDE • Easy, Medium and Hard, so the numbers get sneakier as you do • Every answer shows its method, step by step • A star-shaped tool button that fans out Roots, Prime tricks and a Checklist, for when you're stuck • Eleven divisibility tricks, from "last digit is even" to the chop tests for 7, 13 and 17 • 168 primes under 1,000. Each one lights up the first time you call it • Three looks: Lamp (warm amber in the dark), Marginalia (gilt on a library green) and Phosphor (an 80s terminal), each in light and dark • How to play, plus a short scrolling tour of why primes matter: the atoms of math, Euclid's proof that they never run out, the Ulam spiral, prime-year cicadas and the primes that keep your secrets NO HOOKS No accounts, no ads, no notifications, no tracking. It works offline, and nothing leaves your phone. Play a few, then put it down.