If H and K are subgroups of a group G and if |H| and |K| are relatively prime, prove that $\displaystyle H \cap K = \{1\}$.

I realize that this amounts to proving that $\displaystyle |H \cap K| = 1$, since the intersection of any family of subgroups is a subgroup. However, I don't know how where to go from here. Any help would be greatly appreciated!