We already explained that foods and liquids that touch one of the primary sources of impurity become primary derivatives of impurity. Similarly, if food touched a person or a k'li that had contracted impurity from a source of impurity, that food becomes a second derivative of impurity. Food that touches this secondary derivative is considered a tertiary derivative of impurity and if the tertiary derivative touches a fourth food, it is considered a fourth degree derivative of impurity.
To what does the above apply? To foods. With regard to liquids, by contrast, whether a liquid touches a primary source of impurity, a primary derivative or a secondary derivative, that liquid is considered as a primary derivative. It imparts impurity to another liquid, which in turn can impart impurity to another liquid, even if there is a chain of a hundred. For there is no concept of degrees of impurity with regard to liquids.
What is implied? When wine touched a primary source of impurity or a primary or secondary derivative, the wine is considered as a primary derivative of impurity. Similarly, if this wine touched oil, that oil touched milk, that milk touched honey, that honey touched water, and that water touched wine, in an endless chain, all the liquids are considered as primary derivatives. It is as if each one of them contracted impurity from a primary source. They all impart impurity to keilim. Similarly, if the outer surface of a container contracted impurity from a liquid and other liquids touched that impure outer surface, even if the other liquids are not sacred, the other liquids are considered as a primary derivative of impurity and impart impurity to other containers, making them secondary derivatives of impurity. Needless to say, they impart impurity to other foods and liquids.