Eratosthenes Sieve.

     In the line of natural numbers we mark the first prime number -  2 and cross out every second number after 2. Then we take the first unmarked uncrosed number - in our case it's 3, mark it and cross out every third after it. Then we mark the next first uncrossed number -  5 and cross out every fifth number after it, e.c. It's easy to show, that as a result, all uncrossed numbers will be prime.