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Farmers often have to clear their fields of big stones.
Physicists say it is not so much that big stones work their way up as it is that smaller particles work their way down. Physicists have long known that in a mix of large and small objects, the large ones tend to end up higher while the small ones end up lower. This holds true whether the substance is powder, cornflakes, nuts or fieldstone, and regardless of the texture, shape or weight of the particles. It was not until 1987, however, that scientists at Carnegie-Mellon University in Pittsburgh published the results of a simulation study that appears to explain why this happens. The scientists created a computer model based on particles left to interact with one anothereach other completely at random.