Eratosthenes was a Greek Mathematician, who lived 276 BC to 194 BC in northern Africa.  He studied prime numbers, and is credited with the invention of the method of finding them illustrated on this page, called the Seive of Eratosthenes.

Sieve of Eratosthenes

In this method, numbers are written in a table, such as the one, below.  Beginning with the first prime, 2, all "proper" multiples of 2 -- that is, multiples other than 2 itself -- are crossed off.   The next number not already crossed off is guaranteed to be prime -- 3 in this case.  Continue crossing off the proper multiples of each number.   Since this table is square, when you have finished crossing off all the proper multiples of the primes in the first row, the table will contain only primes.

Start with the number 2.

When all proper multiples of all numbers in the first row are deleted, the table will contain only primes 

The webmaster and author of the Math Help site is Graeme McRae.
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