There are an infinite number of prime numbers. Determining whether or not a number is prime can be hard, even for computers. One way of finding prime numbers comes from Eratosthenes, who was born in 276 BC in Libya. The method is called the Sieve of Eratosthenes.
The method works like this: make a table of all the numbers that you want to check to see if they are prime. Starting with the number 2, cross out all the numbers divisible by 2. Then repeat the process, crossing out numbers divisible by 3, 4 and so on. The numbers left uncrossed will be prime numbers.
This program creates the table of numbers. Each time you press the "Mark!" button, the program will cross out the next set of numbers. To start over, press the "Reset!" button.
Read more about the Sieve of Eratosthenes.