הלכות שאר אבות הטומאה

It is explicitly stated in the Torah that foods and liquids contract ritual impurity, as Leviticus 11:34 states: "Of all foods that will be eaten that were exposed to water will become impure and any beverage which one will drink in any vessel will become impure."
Food that contracted impurity does not impart impurity to other foods according to Scriptural Law. Nor do liquids that contracted impurity impart impurity to other entities according to Scriptural Law. Nevertheless, according to Rabbinic Law, it was decreed that impure food that touched other foods would impart impurity to them. Similarly, if impure foods touched liquids, the liquids are impure. Food does not, by contrast, ever impart impurity to a container that it touches even according to Rabbinic Law. Similarly, the Sages decreed that impure liquids should impart impurity to foods, liquids, and keilim that they touch.

There are no derivatives of impurity that impart impurity to keilim other than impure liquids. This impurity is a Rabbinic decree. It applies provided the original source of the liquid's impurity is a primary category of impurity, whether of Scriptural or Rabbinic origin.
Why did they decree that impure liquids should impart impurity to keilim? Because of a similarity to the liquids that emanate from a zav, which are a primary source of impurity and impart impurity to keilim according to Scriptural Law, as ibid. 15:8 states: "When a zav shall spit on a person who is pure," as we explained.

When the Sages decreed that impure liquids should impart impurity to containers, they decreed that they should impart impurity from their inner surfaces.
What is implied? If impure liquids fell into the inner space of an earthenware container, it becomes impure in its entirety. It is considered as a secondary derivative. If they touch other containers on their inner surface, they become impure in their entirety and are considered as secondary derivatives of impurity. If, however, impure liquids touch the outer surface of a container, whether it be an earthenware container, a k'li that can be purified by immersion, or a metal k'li, only the outer surface becomes impure. The outer surface is considered as a secondary derivative, but its inner surface does not become impure.
In what context does the above apply? To terumah. For sacrificial foods, by contrast, when the outside of a container becomes impure, it becomes impure in its entirety and it is considered a secondary derivative of impurity in its entirety.

We already explained, that foods do not impart impurity to utensils, but that liquids do. Why did our Sages make the above distinction - that when the outer surface of a container was touched by impure liquids, its inner surface does not become impure for terumah? To make it known that the impurity of these keilim is a Rabbinic institution so that terumah and sacrificial foods will not be burnt as a result.

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.

When the outer surface of a container contracted impurity and then foods - even food that are terumah - touched this impure outer surface, the foods are pure. For when only the outer surface of a container contracts impurity, it does not impart impurity to foods with the exception of sacrificial foods. For when the outer surface of a container contracts impurity, it is considered as entirely impure with regard to sacrificial foods, as we explained. Therefore, it imparts impurity to sacrificial foods.

When the outer surface of a container contracted impurity and then foods - even food that are terumah - touched this impure outer surface, the foods are pure. For when only the outer surface of a container contracts impurity, it does not impart impurity to foods with the exception of sacrificial foods. For when the outer surface of a container contracts impurity, it is considered as entirely impure with regard to sacrificial foods, as we explained. Therefore, it imparts impurity to sacrificial foods.