A prime number is a whole number greater than 1 that has exactly two factors. Those factors will always be 1 and the number itself.
A prime number can only be divided by itself and one to give a whole number answer.
The prime numbers up to 20 are: 2, 3, 5, 7, 11, 13, 17, 19
Play Superbug smash! to practise recognising prime numbers