It's been a long time since I posted a Puzzler, but as I was perusing Khan's CS courses this morning (which look really cool!) I came across this fascinating discourse on prime numbers: Sieve of Eratosthenes.
Your challenge today is simple - watch the video (via the link above or embed below) - and once the theory is discussed, do not look at the full solution. Write up a solution in your language of choice and post your answer below. You can also use a Gist or Pastebin link.