Walk along a beach at low tide when the sand is firm and wet. At each step, the sand immediately around your foot dries out and turns white. Why? The popular answer, that your weight ‘squeezes the water out,” is incorrect: sand doesn’t behave like a sponge. So what does cause the whitening?
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Before you step on it. the sand is packed as tightly as it can be under natural conditions. Your weight disturbs the sand, making the grains less efficiently packed. The sand is forced to occupy more volume and rises above the water level. becoming dry and white. The water rises more slowly. by capillary action, so it takes a few seconds or more before the sand gets wet and dark again.
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