Shift
A grain, from one evening to the next evening, experiences a shift. After two evenings, the grain has shifted from one location to another.
Picture the grain in your head with a time-lapse of one evening after another happening in the background. Slowly, the grain shifts.
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うつ
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This shift is caused by the eagle (い) that wants to eat the grains. Every other evening, the eagle swoops down, shifts the grains (eating a few) and then flies off.
Picture the eagle flying down on the second evening, scattering the grains it doesn't eat, shifting their location.