Swelling Sand

Grains of wet sand—in this exhibit or on a beach—become closely packed together. When you press on the sand, the nearby sand grains push one another into a more jumbled arrangement, creating a raised area around your finger. Water flows down through the gaps between the grains, leaving dry sand above. This phenomenon is called the wet sand effect, or dilatancy (die-LATE-un-see), because the sand expands, or dilates, under pressure.

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