A factor is a number that goes into another. The factors of 10 for example are 1, 2, 5 and 10. Prime numbers are a special set of numbers that only have two factors: themselves and 1. An example ...

2 is the only even prime. 2 + 14 = 16. 14 has more than two factors (1, 2, 7, 14). Other pairs of even numbers adding to 16 can be ignored (4 + 12, 6 + 10 and 8 + 8), none are prime. 16 = 1 + 15.