A foundational principle of ritual purity is that agricultural produce is naturally immune to impurity while growing. Seeds and fruits remain entirely protected in their natural state until two conditions are met: they are detached from the earth, and they come into contact with liquid. Only then do they enter a state where they can become ritually impure.
The process of becoming susceptible to impurity is not merely physical; it requires human involvement. Liquid must come into contact with the produce through human will or satisfaction. If rain falls naturally onto harvested crops, they remain immune unless a person actually desired that water. For instance, if someone leaves a bowl outside to be washed by the rain, and the splashing water wets nearby fruit, that fruit is now vulnerable to impurity because the water's presence was wanted. This intention does not even need to come from the owner of the produce; the satisfaction of any person regarding the water is sufficient [תורה תמימה].
While water is the primary example, this rule applies equally to all standard liquids, a principle derived from earlier laws concerning household vessels. The method of contact is irrelevant—whether liquid spills onto garden seeds and human food, or the food drops into the liquid, the result is the same. From a mystical perspective, water specifically possesses this unique ability to open an object to impurity, a concept tied to the hidden spiritual process of fire being drawn out of water [רקנאטי].
The primary approach among commentators is that once produce is wetted with human consent, this susceptibility is permanent. Even if the food later dries out completely, it will instantly become impure if a dead creeping animal touches it. The initial moisture permanently alters the spiritual status of the food, creating a lasting connection to impurity. If the law were to fluctuate based on whether the item was currently wet or dry, the system of purity would become inconsistent and fragmented [רבנו בחיי].
When contact with a carcass does occur, the impurity is absorbed directly by the seed itself, rather than the water becoming impure and acting as a middleman [מלבי״ם, גור אריה]. However, while the seed becomes impure, it does not transfer this impurity further to other foods. Furthermore, only the actual flesh of the dead creature transfers impurity, excluding dry parts like bones, teeth, nails, and hair [חזקוני].
Finally, the vulnerability to impurity extends beyond the edible part of the food to include anything a person uses to handle it. Attached stems, stalks, or the central branch of a grape cluster are treated as part of the fruit. If an impure animal touches these connecting parts, the impurity travels directly into the fruit itself, provided they have not been separated.