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: