Coffee Rings: Why?

First off, glorious, fantastic news for my fellow Cubs fans: general manager Jim Hendry has been fired.

That established... every so often I post a little science project here. Well, here's another one:

1. Drink a cup of coffee and make sure the cup leaves a stain.
2. Sit there watching that stain as it dries.

Yes. That is your science experiment for today. It formed a ring, right? Your question is, why did it form a ring?

"Because the cup's base formed a ring and the coffee just stuck to it?"

...ooookaaaay, there's that too... tell you what, go get a flat-bottom cup and do it again.

"More coffee? Can't I just skull a 5-Hour Energy or something?"

YOU WILL DO AS THE INTERNET COMMANDS YOU TO, CITIZEN.

...

"It formed another ring."

Okay, now why did it do that?

"Because Obama refuses to do something about the illegal Colombians taking the jobs of hardworking taxpayers!"

Holy hell, man. It's COFFEE. The University of Pennsylvania recently- and I'm sure they have some larger purpose for this I'm not thinking of yet- looked into not only why that happens, but how to prevent it.

I'll let them explain.