^How did anyone figure out prime numbers? Around 200 B.C.,
   Eratosthenes figured out an easy way. Make a list of numbers,
   say, 1 to 100. Skip 1, 'cause it's not a prime number(!), so
   keep 2, then remove every 2nd number on the list. Then with 3,
   remove every 3rd number on the list. And so on and so forth.
   Never learned it in school, but it's easy enough for elementary
   school kids to do. This video shows how simple it is. Prime
   numbers are pretty neat, because they're kinda like the
   "skeleton" of our whole number system. -K^