As to the types of food and drink that are susceptible to ritual defilement: Any portion of food that is usually eaten—and, if it is a fruit or vegetable, upon which water has fallen once it was plucked from the ground or its tree—can become ritually defiled. In addition, any of seven specific beverages that can be drunk in any vessel—water, wine, olive oil, blood, milk, dew, and bee honey—can become ritually defiled.
Not only contact with water, but also contact with any of the other just-mentioned beverages that can be drunk in any vessel will render plucked food susceptible to becoming ritually defiled, even if the food had already dried off at the time of defilement.
Thus, you see that ritual defilement can be contracted indirectly, i.e., through a medium that itself is not inherently defiled (in this case, the earthenware vessel), but only contracted defilement from an original source (in this case, the defiling creature). However, only food and drink contract ritual defilement indirectly; people and implements do not.
An article of food that has contracted ritual defilement directly (e.g., through contact with a dead creature) only conveys ritual defilement to other food (by touching it) if its own volume is at least that of an egg (approximately 57 ml or 2 oz).
Anything upon which any part of the carcasses of these aforementioned animals falls will become ritually defiled. Thus, a portable earthenware oven or stove, once they have become ritually defiled as described above, must be demolished—similar to the case of the earthenware vessel described previously—and their pieces reassembled into a new oven or stove if you wish to use them to prepare ritually undefiled food. If, however, you want to use them to prepare ritually defiled food, which you may eat as long as you understand the restrictions that doing so places upon you, you may keep them intact for use in their defiled state. In contrast, an immovable oven or stove that is built into the ground does not contract ritual defilement, even if it is earthenware.
It was just mentioned that water contained in a vessel can become defiled if the carcass of a defiling creature falls into it. But a spring, cistern, or a gathering of water (mikveh) that is built into or hewn out of the ground remains undefiled, even if a defiling carcass falls into it. Furthermore, a ritually defiled person who immerses himself or herself in a spring, cistern, or gathering of water that is built into or hewn out of the ground becomes rid of this defilement, either totally or partially, depending upon the situation. Nevertheless, even someone who is inside such a mikveh when he or she touches the carcass of these creatures will become defiled. In order to be rid of defilement, the person must separate from the carcass and immerse himself again.
Regarding the role of liquids in rendering plant-derived foods susceptible to ritual defilement, the rule is as follows: If any part of the carcass of any of these aforementioned animals falls upon any sown plant while it is still sown, i.e., before it has been uprooted, it remains undefiled.
A Closer Look
Mikveh: In order for water to purify someone or something from ritual defilement, it must be part of a natural body of water, such as an ocean, lake, permanent river, or spring, or it must be directed from a natural source (such as a natural body of water, rain, or snow) into a cistern. Inasmuch as the entire body must be immersed at once, the minimum volume of a cistern-mikveh is determined to be that held by a parallelepiped 3 cubits long, 1 cubit wide, and 1 cubit deep, which in turn is roughly 40 seah (approximately 332 liters or 88 gallons). There are many additional rules governing the construction of a mikveh, what type of water may be used to fill it, how the water must be directed into the cistern, and so on.
