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（单词翻译）
Why is that? It’s because the edges of the spill are where the fluid is at its thinnest. A spill sitting on a flat surface is something like a pancake. Big in the middle and thin on the rim1. As the thin edges evaporate, they are replenished2 by liquid and particles of coffee from the denser3 interior. That starts an outward push going in the puddle4. The more it dries, the more particles get drawn5 out to the edges. When all of the liquid has evaporated we’re left with a ring of coffee particles making a stain.
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