when god sends down water from the sky and floods run through the valleys certain quantities of foam rise on the surface of the flood water. this is similar to that foam which rises when you expose something to the heat of a fire to manufacture ornaments or for other reasons. to god truth and falsehood are like these examples. the foam disappears but what is profitable to the human being stays in the land. thus does god coin his parables.
أنزل من السماء ماء فسالت أودية بقدرها فاحتمل السيل زبدا رابيا ومما يوقدون عليه في النار ابتغاء حلية أو متاع زبد مثله كذلك يضرب الله الحق والباطل فأما الزبد فيذهب جفاء وأما ما ينفع الناس فيمكث في الأرض كذلك يضرب الله الأمثال