Why does coffee stain?

Why is a coffee stain/ring only made at the top?

• Why is a coffee stain only made at the top of a cup? It's like a solid ring that goes around. Even if there are other rings in the cup, why isn't it all brown? It's usually most prominent at the top. Is coffee more still near the top? allowing it to settle and stain easier than the coffee bellow the top? and, if there are rings bellow the top, (say you didn't get to drink that much, you were tired of coffee and decided to dump the rest of it) Why is that part a ring? If this sounds like a stupid question, it probably is. I haven't slept in 21 hours, but I'm still curious. I ain't trollin XD

• Answer:

  Somebody asked this question a few months ago and I tried to research to have concrete proof of why and couldn't find it. What I recall is that liquid in a container creates a little sort of bubble at the top, where water touches air and a sort of pocket is created, the two meeting each other. The reason the ring forms is because of that pocket and how solids collect at that point where water/liquid meets air - and it may be from the difference in pressure air against liquid. It happens with any liquid, milk, juice, and even in containers, and not to get gross, it happens in the toilet bowl. I remember learning about the little bubble taking a cooking class in high school, a long time ago, and it was explained that in measuring liquids you have to measure to the bottom of the little bubble because air was in that pocket. For the life of me I can't find any reference online to explain the phenomenon accurately and looked again to be sure. edit: came up with what I believe is the most plausible explanation - surface tension of water and air. It occurs when the surface of a liquid resists another force - which would be air. When surface tension occurs circular water droplet occur, bubbles. On a larger surface the 'bubble' becomes a pocket across the surface of the water, where force is exerted water against air. The water and solids in water pressing against the air essentially form a line of tension against each others and that's likely how the surface line may occur, because of the pressure of the two surfaces meeting. You might want to ask in the physics section - I've just followed a thought I had and found some references to support that. and here's wikipedia's entry for surface tension http://en.wikipedia.org/wiki/Surface_tension