Way back in ancient Greece, a mathematician called Eratosthenes
( c. 276 BC - c. 195 BC ) came up with a cool way to find prime numbers. By the way, don't try to say his name with a mouthful of soda crackers! Especially not if I'm standing in front of you.
Here's his method:
The Sieve of Eratosthenes:
You start by making a grid of the numbers 1 - 100:
First, we cross out 1, since we know it isn't prime: