What are the common factors of 9 and 21?

Oh, dude, the common factors of 9 and 21 are the numbers that can divide both of them without leaving a remainder. So, the factors of 9 are 1, 3, and 9, while the factors of 21 are 1, 3, 7, and 21. The common factors between them are 1 and 3. Cool, right?