Next time you spill a little, let the coffee dry before wiping it up. You’ll notice that after it dries there is a darkish ring around the very edge of the spill, whereas the middle disappears almost entirely. What’s going on here? Robert Deegan and Sydney Nagel at the University of Chicago looked into this phenomenon, and they discovered something pretty interesting. The coffee puddle leaves a ring because it doesn’t simply dry where it sits. As the spill begins to dry, it actually creates its own outward motion, drawing the insides of the puddle toward the edges.