What is the green stuff that builds up on brass door handles?

[Michalywood]

This may seem like an odd question. I've tried searching the web to no avail. It's basically green stuff, plasticine-like, and builds up in the pattern indents in the parts that get touched the most. I've always wondered where it comes from.

[Jorge]

It's just the metal oxidising is it not?

http://en.wikipedia.org/wiki/Patina