Why are things more vertical when they are poured onto a slope?

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Let’s say that I pour rice out of a cup. Why are the grains not randomly organized? The grains at the top are random, but the grains that fall on the slopes have this radial pattern.

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Anonymous 0 Comments

Rice is not a perfect sphere and it is not sliding down a perfectly smooth surface. It is an ovoid shape sliding down a lumpy surface of other rice.

The rice is more easily able to slide between two other grains in its long dimension. If it is somewhat aligned with that gap its shape will tend to make it line up as it slides. As a result rice that slides down a pile of rice will tend to be aligned along the slope.