dugdha yena amla-yoge dadhi-rupa dhare
dugdhantara vastu nahe, dugdha haite nare
dugdha—milk; yena—as; amla:yoge—in association with a sour substance; dadhi-rupa—the form of yogurt; dhare—takes; dugdha-antara—something other than milk; vastu—substance; nahe—is not; dugdha—milk; haite—to be; nare—is not able.
"Milk is transformed into yogurt when it associates with a yogurt culture. Thus yogurt is nothing but milk, but still it is not milk.
Of the three deities supervising the creation, maintenance and dissolution of the universe, Lord Visnu is never separate from the original Visnu. However, Lord Siva and Brahma, due to their association with maya, are different from Visnu. Visnu cannot be transformed into any form of material energy. Whenever there is association with maya, the personality involved must be different from Lord Visnu. Therefore Lord Siva and Lord Brahma are called guna-avataras, for they associate with the material qualities. The conclusion is that Rudra is a transformation of Visnu, but he is not exactly Lord Visnu. Therefore, he does not come within the category of the visnu-tattvas. Thus he is inconceivably one with Visnu and different from Him. The example given in this verse is very clear. Milk is compared to Visnu. As soon as milk touches a sour substance, it becomes yogurt, or Lord Siva. Although yogurt is constitutionally milk it cannot be used in place of milk.
Link to this page: https://prabhupadabooks.com/cc/madhya/20/309