CC Madhya 20.309
dugdhāntara vastu nahe, dugdha haite nāre
“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 Viṣṇu is never separate from the original Viṣṇu. However, Lord Śiva and Brahmā, due to their association with māyā, are different from Viṣṇu. Viṣṇu cannot be transformed into any form of material energy. Whenever there is association with māyā, the personality involved must be different from Lord Viṣṇu. Therefore Lord Śiva and Lord Brahmā are called guṇa-avatāras, for they associate with the material qualities. The conclusion is that Rudra is not exactly Lord Viṣṇu but rather a transformation of Viṣṇu. Therefore, he does not come within the category of the viṣṇu-tattvas. Thus he is inconceivably one with Viṣṇu and different from Him. The example given in this verse is very clear. Milk is compared to Viṣṇu. As soon as milk touches a sour substance, it becomes yogurt, or Lord Śiva. Although yogurt is constitutionally milk, it cannot be used in place of milk.