What is a prime number?
A prime number has only two factors, 1 and the number itself.
2 is a prime number, because its only factors are 1 and 2.
The number 2 is the only even prime number.
15 is not a prime number, because it has four factors (1, 3, 5 and 15).
Watch out for odd square numbers such as 9 or 25 - it is easy to forget that 9 actually has 3 factors (1, 3 and 9) and so is not a prime number.
1 is also not a prime number, as it only has one factor - the number 1.