elementary number theory - Prove if 2 divides $a^2$, then 2 divides $a$.

If 2 divides $a^2$, then 2 divides a.

I know that 2 divides $a^2$ means there is some integer $n$ such that $a^2 = 2n$,
and similarly, 2 divides $a$ means there is some integer $m$ such that $a = 2m$

I thought I could rewrite $a^2 = 2n$ into this $= a = 2(n/a)$ but I don't think that helps, because I'm not sure $n/a$ is an integer.

Thank you for any help!