Prime numbers are divisible only by 1 and themselves. They serve as the “atoms” of mathematics, capable of decomposing other numbers into factors (like 12 = 2 × 2 × 3).

9 is not a prime number. It can be divided by 3 as well as 1 and 9. The prime numbers below 20 are: 2, 3, 5, 7, 11, 13, 17, 19. Don't forget: the number 1 is not thought of as a prime number.

The basic idea behind the algorithm is to take a number, n, and through a series of passes, eliminate the non-prime numbers. What remains when you’re done is the set of primes you are after.

The online computer game “Is this prime?” tests a player’s knowledge of prime numbers—and just surpassed 2,999,999 attempts. Give it a whirl.

Prime numbers can only be divided by themselves and 1. Find out how to work out if a number is a prime in this Bitesize Primary Second Level Maths guide.