An integer which has two, and only two, factors is a prime number.

• 0 has no factors, and is not a prime number

• 1 has only 1 factor (1), and is not a prime number

• 2 has two factors (1 and 2) and is a prime number. It is the only even prime number

• 3 is a prime number, as it has factors of 1 and 3

• 4 is not a prime number, as it has three factors: 1, 2 and 4.

The other prime numbers below 20 are: 5, 7, 11, 13, 17 and 19.

To work out whether a number is prime, divide by each of the prime numbers starting with 2.  If it is not divisible by any of these numbers, it is a prime number.