Ok, if you've read the first blog on prime numbers, you know some rules that help you reduce the number of numbers you have to check to see if a number is prime.
One more rule that may help is the rule that:
All prime numbers are either +1 or -1 from a multiple of 6. Not all +1's and -1's are primes, but all primes are +1 or -1.
What this rule does is move you one away from all the numbers that can be divided by 2 or 3.
Here's another chart of what numbers you have to check as divisors for all primes up to 400.
This chart doesn't prove it, but it gives you hint of a basic rule of all numbers:
Every number is either prime or can be factored (divided) into primes.
This is true just from the way we define prime numbers