On some beaches around the world, dry sand makes a singing, squeaking, whistling, or screaming sound if a person scuffs or shuffles their feet with sufficient force. The phenomenon is not completely understood scientifically, but it has been found that quartz sand does this if the grains are highly spherical. It is believed by some that the sand grains must be of similar size, so the sand must be well sorted by the actions of wind and waves, and that the grains should be close to spherical and have surfaces free of dust, pollution and organic matter. The "singing" sound is then believed to be produced by shear, as each layer of sand grains slides over the layer beneath it. The similarity in size, the uniformity, and the cleanness means that grains move up and down in unison over the layer of grains below them. Even small amounts of pollution on the sand grains reduce the friction enough to silence the sand.