Description:The bowl photographed above contains wet sand. When tapped on its side, the grains of sand settle with their sides so close that water cannot flow into the cracks due to its surface tension. The surface of the sand looks wet. When you push down on the sand with a foot, as though walking on the beach, the spacing between sand grains opens up sufficiently that water can flow down into the space. The sand in the vicinity of the foot appears dry. This effect causes the sand to appear lighter in color by your footprints at the beach.
This effect is known as "dilatancy."