Chapter 13, Verse 18

He sends down water from the sky, so that valleys flow according to their measure, and the flood bears on its surface swelling foam. And from that which they heat in the fire, seeking to make ornaments or utensils, comes out a foam similar to it. Thus does Allah illustrate truth and falsehood. Now, as to the foam, it goes away as rubbish, but as to that which benefits men, it stays on the earth. Thus does Allah set forth parables.