What is the least common factor of 6 and 18?

The factors of 6 are 1, 2, 3, and 6.

The factors of 18 are 1, 2, 3, 6, 9, and 18.

The common factors of 6 and 18 are 1, 2, 3, and 6. Therefore, the least common factor (the smallest common factor) is 1.