Number Theory · 6th Grade
Every whole number is either prime — or built entirely from primes. That makes primes the atoms of arithmetic.
Imagine you had to describe every number using just a small set of "building blocks." It turns out, you can. The building blocks are called prime numbers — numbers that can't be broken down any further.
A prime is any whole number greater than 1 that has no divisors except 1 and itself. So 7 is prime (nothing divides it evenly), but 6 is not — because 6 = 2 × 3.
Around 200 BCE, a Greek mathematician named Eratosthenes invented a brilliant method to find all primes at once. He imagined passing all numbers through a sieve — composites fall through the holes, primes stay behind.
(The same Euler who solved the Königsberg bridge puzzle also proved that primes go on forever — they never stop, no matter how far you count. Euler shows up everywhere!)
Cross out composites step by step. Whatever's left is prime.
Six numbers. Trust your instincts.
Is 7 a prime number?
Enter any number up to 1,000 and we'll check it.
