A prime number is a whole number greater than 1 that has only two divisors: 1 and itself. In other words, it can only be divided by 1 and the number itself without leaving a remainder. Let's take the number 7 as an example. The factors of 7 are 1 and 7 because these are the only two numbers that can be multiplied together to get 7. So, 7 is a prime number.
Now, let's think about the number 6. The factors of 6 are 1, 2, 3, and 6. Since 6 has more than two factors, it is not a prime number. Numbers like 6 that have more than two factors are called composite numbers. To recap, prime numbers have exactly two factors, while composite numbers have more than two factors.
To create a list of prime numbers, we start with the smallest prime number, which is 2. Then, we check each consecutive number to see if it is prime. We can use a method called the Sieve of Eratosthenes to find prime numbers. This method involves crossing out the multiples of each prime number to reveal the prime numbers left. Let's create a list of prime numbers up to 20 using this method:
2 (since 2 is the smallest prime number)
3 (cross out multiples of 3 such as 6, 9, 12, and 15)
5 (cross out multiples of 5 such as 10 and 15)
7 (cross out multiples of 7 such as 14)
11 (cross out multiples of 11)
13 (cross out multiples of 13)
17 (cross out multiples of 17)
19 (cross out multiples of 19)