Prime numbers

Prime numbers are special numbers, greater than 1, that have exactly two factors, themselves and 1.

19 is a prime number. It can only be divided by 1 and 19.

9 is not a prime number. It can be divided by 3 as well as 1 and 9.

The prime numbers below 20 are: 2, 3, 5, 7, 11, 13, 17, 19.

Don't forget: the number 1 is not a prime number as it only has one factor.