soft question - How many different representations of pi can we come up with?

Let me explain: a friend of a friend is opening a new pizza restaurant called "Pi", and he's looking to decorate his walls with pi-related material: formulas, equations, theorems w/ proof, diagrams, etc. Any suggestion is welcome, so long as it meets these two criteria:

1. It has to be mathematically correct.


2. It has to be either a representation of pi itself or lead directly to a representation of pi.

So for example, this is okay: $sum_{n=1}^{infty} frac{1}{n^2}$ (because it equals $frac{pi^2}{6}$)
But this is not: $frac{22}{7}$.

How many can we come up with?