Why are all the grains of sand on the beach the same size (roughly)?
I am not too clear about the precise dimensions in which this effect operates, but when the stones get ground down to a certain size grain, the surface tension of the water surrounding them begins to have a relatively significant effect, and actually ‘protects’ the grain from further erosion. That is, the surface tension of the water on the surface of the grain forms an envelope around the grain. It IS an astonishing effect! and it means that not only are all of the grains of sand on a single beach of very similar size, but that in general, grains of beach sand tend to be similar in EVERY beach. If wind is a significant factor, then this protective action of water is no longer relevant: the grains will continue to erode, and wind is always something of a factor, so sand does not quite stop getting finer once this size is reached, but it does account for the surprising uniformity in size.
