He sends down water from the height causing the riverbeds to flow according to their measure, and the flood bears on its surface swelling foam. And, when they use fire to refine metals for their jewelry or equipment, foam is produced like it. In this way Allah is citing for you the example of the Truth and the Falsehood. For, as far as the foam is concerned, it passes away as scum upon the banks. While that which is of benefit to mankind, abides on earth. (The real existence on earth is of the person or the system that benefits humanity). This is how Allah uses analogies for you to understand.