He sends down water from the sky and river-beds fill up and flow according to their size, and the floodwater carries with it an increasing layer of scum; a similar kind of scum comes from what you heat up in the fire, when you desire to make jewellery or other things. That is how Allah depicts the true and the false. As for the scum, it is quickly swept away. But as for that which is of use to people, it remains behind in the ground. That is a metaphor which Allah has made