13.17 . He sendeth down water from the sky , so that valleys flow according to their measure , and the flood beareth ( on its surface ) swelling foam from that which they felt in the fire in order to make ornaments and tools riseth a foam like unto it thus Allah coineth ( the similitude of ) the true and : the false . Then , as for the foam , it passeth away as scum upon the banks , while , as for that which is of use to mankind , it remaineth in the earth . Thus Allah coineth the similitudes .