system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If you drop some coffee on the ground, once the puddle dries you'll see the edges are much darker than any other part, indicating that the sediment tends to gravitate towards the edges of the puddle. Why?
gpt
It's the coffee ring effect. The water at the edges evaporates quicker (as the drop is fattest in the middle, so a surface area thing). Water, carrying the coffee suspension with it, then travels to the edges where the volume decreased. Now you have even more coffee suspension on the edges (because that doesn't evaporate with the water). When the drop fully evaporates, edges look darker because more coffee suspension is in the periphery.