Sieve of Eratosthenes

The Sieve of Eratosthenes is a way of finding all of the prime numbers (a number that is only divisible by itself and one) up to a certain number. It starts by making a list of all of the numbers starting with 2 and ending with the number you chose. Then, it finds the first number in the list that has not been crossed out, which the first number is always 2. It sees 2 is not crossed out and it is a prime number (so it can't be made by multiplying other numbers), so it crosses out every second number in the list. That means it crosses out all of the multiples of 2 (4, 6, 8, 10, 12, etc.). It then finds the next number that has not been crossed out, which is 3, and it does the same as it did for 2 (crosses out all of the multiples of 3: 6, 9, 12, 15, etc.). It continues to do this for every number in the list, until it gets to the number you chose. When it is finished, the numbers that are not crossed out are all the prime numbers up to the number that you chose.