Subject: Where does Pi appear in nature?

Where does Pi appear in nature?
I have heard that Pi (3.1415926...) appears in all sorts of
unexpected places in Physics / Biology / Chemistry and nature 
generally.  Someone told me it even appears in an equation 
describing space-time in general relativity.  Pi is also used in 
some areas of statistics I believe. 

I would love to know where else this ubiquitous 
number appears in equations that describe the world around us.

And WHY does it appear in all these strange places?

Thanks!

Nick.

Re:Where does Pi appear in nature?