Take $e$ and $\pi$. Scale $e$ to $a$, $\pi$ to $b$ somehow such that $b-a$ is almost zero.
Then there must be a rational $q$ between $a$ and $b$: $a<q<b$. How would you find this $q$?
If $a$ and $b$ were rationals, I would just say: take $\frac{a-b}{2}$ and define that as $q$. But, if $\frac{a-b}{2}$ is irrational, how to find a rational $q$ between $a$ and $b$?