The phrase "sand grains" refers to the tiny particles of sand that make up a beach or desert. These grains are small and often smooth, and when many of them come together, they form large sandy areas. Full definition
At just the right mix, when formed into a carefully crafted castle on the beach, the water helps to bind together the individual sand grains, maintaining the structure's shape. (science.howstuffworks.com)
When this happens, the larger sand grains typically lose electrons to the lighter dust particles, giving the dust a negative charge. (sciencemag.org)
The dust particles are blown higher into the air more readily, whereas the now positively charged sand grains usually remain closer to ground level. (sciencemag.org)