Find the greatest common factor of 180 and 168.
If a number $d$ divides into both $180$ and $168$, it must also divide into their difference. Thus, $d$ will be divisible by $180 - 168 = 12$. We notice that $12$ divides into both $180$ and $168$, so it follows that $\boxed{12}$ must be the GCF of $180$ and $168$.