He sends down water from the sky and the valleys flow according to their (respective) capacity. Then the floodwater carries the scum raised to the surface. And the items that are heated in the fire for making ornaments or other articles also have foam that comes up the same way. Thus Allah illustrates examples of the Truth and falsehood. So, (whether produced by water or by fire) the foam or scum passes away useless but what is beneficial to the people endures in the earth. That is how Allah illustrates examples.