As long as beans have been brewed, and cups have been sipped, and sloshed, they’ve created those distinctive splotches and circles we know so well: coffee rings.
I have no idea why this is, but for some reason I always have managed to watermark many, if not most of my books, with coffee stains. In many cases I managed to do so only minutes after opening the book for the first time. Somehow I considered them already something like a trademark of mine, as my books and materials where easily to be identified.
As a matter of fact, in a previous version of this blog, I actually had included a coffee stain in the design as well. Luckily these times are over by now.
If you are interested into the physics of coffee stains or rings, I recommend this article on Radiolab which comes just in time, when I really need a coffee.