elementary set theory - Explicit bijection between $mathbb Q$ and $mathbb Z times mathbb Z$?

Any idea of an explicit bijection between $\mathbb Q$ and $\mathbb Z \times \mathbb Z$? Even if I think of rational elements as $\frac {m}{n}$, sending them to $(m,n)$ won't work, because all pairs $(m,0)$ don't have a source...

Any hints would be much appreciated!