Take a look at your multiplication table. (click here to open it in a separate window, so you can keep reading this page). Notice that some numbers appear quite a bit in the non-bold areas. for example, how many times does 12 appear? 16? 24? Then notice that some numbers never appear except in the bold area. Find a couple of these numbers. Why do you think those numbers never appear in the non bold area of the table?

There is a name for those numbers - they're called prime numbers (or just primes, for short). They are numbers that cannot be evenly divided by any other number. (dividing by one doesn't count, because you just get the same number again.)

How can you know if a number is prime or not? Is there an easy way to figure it out? This page has a program that figures out prime numbers - you can look at them here. Click on the links to see either the next higher, or next lower prime.
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Check your multiplication table, and see if you can find any of those numbers not in bold.

At 0, 0% of all numbers are prime.

The rapidly changing number up there is counting through primes, starting at the beginning. It is also keeping track of what percentage of numbers up to that point, out of all the counting numbers, are primes. It starts off pretty high, but notice how soon it starts leveling off.