It is Prime Day. Rather than giving your money to Amazon, spend your Prime Day
learning about prime numbers. A
prime number is a number that can only be divided evenly by itself and 1. By
definition, 1 is not a prime number.

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.