ויקרא, פרק י״א, פסוק ל״ד

The laws of purity establish clear boundaries around what a person consumes. When an earthenware vessel becomes impure, specific rules dictate what materials within its airspace are affected. The primary approach among commentators is that this applies exclusively to food and drink; other objects or human beings within the vessel's airspace remain unaffected.

To contract impurity, food must be intended for human consumption. Animal feed is excluded unless a person mentally designates it for human use, which then renders it susceptible [רמב״ן, תורה תמימה, מלבי״ם]. Similarly, spoiled or rotten food is excluded. Once food decays to the point that it is no longer fit for human consumption, it permanently loses its capacity to become impure [רמב״ן, תורה תמימה, רלב״ג, מלבי״ם, הכתב והקבלה].

A fundamental condition for impurity is that dry food is completely immune until it is intentionally prepared for consumption. God established that contact with water marks the beginning of this preparation [רשב״ם, חזקוני]. This principle dictates that food can never become impure unless it has been wetted at least once. Once this occurs, even if the food dries completely, it remains permanently susceptible [רש״י, מזרחי, בכור שור]. This process requires human intention or desire [הכתב והקבלה, פרדס יוסף, בכור שור], and both the food and the water must be detached from the ground [רש״י, רלב״ג, חזקוני]. Furthermore, the liquid must be drawn into a vessel, as water from natural springs or pools does not trigger this susceptibility [מזרחי, מלבי״ם, חזקוני].

This mechanism extends beyond water to other beverages, provided they are fit for human consumption and not spoiled [תורה תמימה, רלב״ג, מלבי״ם]. However, not just any fluid qualifies. The law recognizes only seven specific liquids: water, dew, wine, oil, blood, milk, and honey. Fruit juices, such as pomegranate or strawberry juice, do not possess the capacity to prepare food for impurity [רש״י, מזרחי, רלב״ג, חזקוני, רד״צ הופמן].

Beyond their role as a catalyst for solid food, there is a broad dispute regarding whether these liquids themselves contract impurity by Biblical law. Some maintain that liquids can both contract and transfer impurity to other items. Others argue they can become impure but cannot transfer that impurity. A third perspective suggests that liquids do not contract impurity on a Biblical level at all, and their mention serves only to explain how they prepare solid food [מזרחי, תורה תמימה, מלבי״ם, רש״ר הירש, רד״צ הופמן]. Because the characteristics of food and drink impurity differ significantly, the laws regarding their respective impurities are treated as distinct categories [תורה תמימה, מלבי״ם].

Finally, there is a quantitative requirement based on the standard capacity of the human throat, estimated by the Sages to be the volume of a chicken egg [רש״י, רמב״ן, תורה תמימה]. Commentators debate how this measurement is applied. One approach asserts that food can contract impurity at any size, even as small as a mustard seed, and the egg-sized volume is only necessary for the food to transfer its impurity to other items [רש״י, רמב״ן, תורה תמימה בשם הרמב״ם, דברי דוד]. Conversely, others hold that the egg-sized volume is an absolute minimum requirement, necessary not only to transfer impurity but to contract it in the first place [תורה תמימה בשם התוספות, מזרחי, משכיל לדוד].