Almost any finely divided organic substance becomes an explosive material when dispersed as an air suspension; hence, a very fine flour is dangerously explosive in air suspension. This poses a significant risk when milling grain to produce flour, so mills go to great lengths to remove sources of sparks. These measures include carefully sifting the grain before it is milled or ground to remove stones, which could strike sparks from the millstones, and the use of magnets to remove metallic debris able to strike sparks.