How Can a set of numbers have a GCF greater than 1?

Ex: 30 and 42

The factors of 30 are:

1, 2, 3, 5, 6, 10, 15, 30

The factors of 42 are:

1, 2, 3, 6, 7, 14, 21, 42

The common factors are:

1, 2, 3, 6

The Greatest Common Factor:

GCF = 6