The drip coffee machine in our graduate parlour regularly gets left on.
Sometimes for a period of 2-4 days.
Quite possibly longer.
At the end of this kind of blitzkrieg, there's often a deep, baked, pure essence of coffee death left at the bottom of the jug.
Now...
What interests me is that when I wash the jug out, the coffee tar froths up. The amount of foam and the bubbliness have an... um... surface tension? Or a shape / form / appearance JUST like washing up liquid.
Any ideas why?