OPINIONS: Rebbi Yonasan (end of 24b) teaches that a Kli Cheres (an earthenware vessel) that is Tamei causes a food item that enters its airspace to become Tamei ("Metamei b'Aviro"), as derived from the verse, "Everything that is inside it shall become Tamei"(Vayikra 11:33) -- even something as small as mustard seeds.
It seems clear from the Gemara that mustard seeds are Mekabel Tum'ah even though each one is less than the size of a k'Beitzah. What is the minimum size necessary for a food item to become Tamei?
(a) RASHI in Pesachim (33b, DH b'k'Beitzah) quotes the Toras Kohanim which implies that even the smallest piece of food is Mekabel Tum'ah mid'Oraisa. (In order to cause something else to become Tamei, however, the food item must be at least the size of a k'Beitzah, as Rashi here (DH v'Afilu) says.) The Gemara here supports this opinion.
(b) TOSFOS in Pesachim (33b, DH l'Eimas) maintains that the Toras Kohanim is teaching only an Asmachta for a Halachah d'Rabanan. A piece of food smaller than a k'Beitzah is Mekabel Tum'ah only mid'Rabanan. Mid'Oraisa, the food must be at least a k'Beitzah in order to become Tamei. TOSFOS asserts that Rashi himself in Chulin (82a, DH v'Amar) retracted his opinion and agrees that a food item must be at least a k'Beitzah in order to become Tamei. Tosfos here (24b, DH ha'Torah) explains that when the Gemara says "even mustard seeds" become Tamei in the airspace of a Kli Cheres, it refers only to Tum'ah mid'Rabanan, and mid'Oraisa the Kli Cheres is Metamei everything in it, even eggs.
(c) The RASHBA (Shabbos 91a) maintains that a food item that is less than a k'Beitzah does not become Tamei at all, even mid'Rabanan. (Z. Wainstein)
OPINIONS: The Torah teaches that a Kli Cheres is unlike other types of vessels; it becomes Tamei when a Tamei object enters its airspace, but not when a Tamei object touches the outside of its wall. The same distinction applies with regard to how a Kli Cheres transmits Tum'ah. A Kli Cheres transmits Tum'ah to objects that enter its airspace, while other vessels transmit Tum'ah only through contact with an object.
The Gemara points out that the Torah mentions the word (or a variation of the word) "Toch" ("inside") four times, and it teaches what is derived from each occasion of the word "Toch." One of the laws derived from this word is that only the inside of a Kli Cheres becomes Tamei, and not the "inside of the inside, even a Kli Shetef." A "Kli Shetef" refers to a vessel made of wood, metal, or bone, which can become Tahor by being immersed into a Mikvah (in contrast to a Kli Cheres). It is called a "Kli Shetef," or "washable vessel," since it can be immersed in a Mikvah and made Tahor, unlike a Kli Cheres which can become Tahor only by being broken.
What does the Gemara mean when it says that the "inside of the inside" of a Kli Cheres does not become Tamei, and not even the "inside of the inside" of a Kli Shetef?
(a) RASHI (DH Tocho v'Lo Toch Tocho) explains that the "inside of the inside" refers to a case in which a Kli Cheres has another vessel inside of it, and the inner vessel's rim extends above the rim of the Kli Cheres. Rashi explains that when a Tamei object is in the airspace of the outer vessel (between the wall of the outer vessel and the wall of the inner vessel) but not in the airspace of the inner vessel, foods in the inner vessel do not become Tamei, because they are in the "inside of the inside" of the Kli Cheres.
Why does the Gemara add that this apples "even" when the inner vessel is a Kli Shetef? Rashi explains that when the inner vessel is also a Kli Cheres which becomes Tamei only from the inside (and not from the outside), it is obvious that the food inside of it does not become Tamei. Since the Tum'ah is presently outside of the vessel, it is logical that the wall of the inner vessel is considered a separation between the food inside of it and the Tum'ah outside of it. The Gemara is saying that even when the inner vessel is a Kli Shetef, which can become Tamei when a Tamei object touches the outside of its wall, its wall still serves as a separation between the food inside of it and the Tum'ah in the airspace outside its wall.
Rashi adds that the Gemara's question involves only the food inside the inner vessel. The inner vessel itself does not become Tamei in any situation, as the Gemara teaches in Pesachim (20a).
TOSFOS (DH v'Afilu Kli Shetef) challenges Rashi's interpretation. One of his questions is that once the Gemara in Pesachim (20a) teaches that ordinary vessels do not become Tamei by being in the airspace of a Kli Cheres, it is logical that even if the inner vessel is a Kli Shetef, the vessel's wall should serve as an effective separation between the food inside of it and the Tum'ah in the Kli Cheres. What does the Gemara mean when it says that this applies "even" to a Kli Shetef? If this is logical, why is a verse needed to teach it?
(b) Tosfos therefore explains that the Gemara is teaching the following law. There is a rule that "Lo Minah Lo Machriv Bah" -- an object of one type cannot "ruin" the effectiveness of an object of a different type. According to this rule, a Kli Shetef, which is made from a different material than a Kli Cheres, should not be able to obstruct the effectiveness of the Kli Cheres in making whatever is inside of its walls become Tamei, and the food inside the Kli Shetef (which is inside the Kli Cheres) should become Tamei as if the food was inside of the outer, Tamei vessel. The Gemara here derives from the verse of "Toch" that even a Kli Shetef is considered to have its own airspace.
This seems to be the intent of the Gemara in Zevachim (3b; see Insights to Zevachim 3:2). The Gemara there teaches that even though a Korban Chatas that is slaughtered with intent to be a different Korban is invalidated and cannot be offered at all, if it is slaughtered with intent to be eaten as Chulin, the wrong intent does not invalidate the Korban. The Gemara explains that this is because "Lo Minah Lo Machriv Bah" -- having intent to slaughter the animal as an entirely different entity, with a thought that does not relate to the status of a Korban, does not have any effect, and is considered as if it is slaughtered "Setama" and is valid. The Gemara then asks that the same principle should apply to the laws of a Kli Cheres. When there is a Kli Cheres that is Tahor situated inside of an outer Klei Cheres that is Tamei, and the food enters the inner Kli Cheres, the food does not become Tamei. This is because the inner Kli Cheres (the rim of which protrudes above the rim of the outer Kli Cheres) protects the food from the airspace of the outer Kli Cheres which is Tamei. The Beraisa teaches that even if the inner Kli is not a Kli Cheres, but a Kli Shetef, the food in the inner Kli still remains Tahor. The Gemara asks that the food in the inner Kli should not be Tahor according to the principle that "Lo Minah Lo Machriv Bah," since a Kli Shetef is an entirely different type of utensil, and therefore its presence in the Kli Cheres should be disregarded.
The Gemara there answers that a special verse of "Toch" teaches that the food inside the inner Kli, even in a Kli Shetef, remains Tahor. The Gemara there apparently understands that, logically, only a Kli Cheres should be able to interrupt the airspace of another Kli Cheres, even without the verse of "Toch." The verse of "Toch" is necessary only to include a Kli Shetef.
(c) The RAMBAN explains that the Gemara is not teaching that the inner vessel should serve as a separation because the vessel itself does not become Tamei. As Tosfos implies, if this would be logical, then there would be no need for a verse of "Toch." Rather, the inner Kli does not become Tamei only because the verse states that it is considered a separation. The reason why this cannot be derived through logic is that the entire concept of Tum'ah of the airspace of a Kli Cheres is a novel concept. Once the Torah teaches the concept of Tum'ah of the airspace of a Kli Cheres, one would have assumed that anything in its airspace becomes Tamei, even something inside of an inner vessel (that remains Tahor) inside of the Kli Cheres! The verse of "Toch" teaches that this is not true; the "inside of the inside of the vessel" does not become Tamei. Since the Torah excludes this area of the vessel from the Tum'ah of the airspace, this exception applies regardless of whether the inner vessel is made from Cheres or from another material.
When the Gemara says "even" a Kli Shetef, it means merely that all types of inner vessels serve as a separation, and that the Torah is not teaching a specific Halachah about the Kli Shetef which it did not need to teach about a Kli Cheres. Both an inner Kli Cheres and an inner Kli Shetef protect their contents from the Tum'ah of the airspace of the outer vessel by cutting off their contents from the airspace of the outer Kli Cheres. (However, the Ramban agrees that Tosfos' explanation is the primary explanation of the Gemara.) (Y. MONTROSE)
OPINIONS: The Mishnah states that Klei Matchos, metal utensils, are Mekabel Tum'ah. The Torah lists six types of metal utensils -- utensils made from gold, silver, copper, iron, tin, and lead -- that become Tamei and that can be made Tahor through immersion in a Mikvah (Bamidbar 31:22). The fact that the verse mentions six different types of metals and does not use a general term for all of them (such as "utensils made of metal") implies that only these metals become Tamei and require Tevilas Kelim, and one cannot derive through a Binyan Av that other metals become Tamei and require Tevilah.
Aluminum is a new metal that was discovered relatively recently. It was unknown at the time the Torah was given and during the time of the Gemara. Does the verse that list the six metals exclude other metals such as aluminum?
(a) RASHI in Rosh Hashanah (beginning of 19b, DH va'Chachamim) writes that the only types of metal that can become Tamei mid'Oraisa are the six metals listed in the verse. No other type of metal can become Tamei mid'Oraisa. (Rashi there implies that had it been possible to derive a Binyan Av from these metals, even glass would have been included, since it, like metal, can be molten; see RASHASH there.)
Similarly, the TIFERES YISRAEL (in his introduction to Seder Taharos, #44) quotes the VILNA GA'ON who asserts that only the six metals mentioned in the Torah are Mekabel Tum'ah.
(b) However, the TIFERES YISRAEL himself disagrees and maintains that any metal which has similar qualities to those mentioned in the Torah is considered metal with regard to Tum'ah and Taharah. Similarly, the ARUCH HA'SHULCHAN rules that all metals require Tevilah mid'Oraisa in order to become Tahor, and they require Tevilas Kelim when acquired from a Nochri.
RAV MOSHE FEINSTEIN zt'l (IGROS MOSHE YD 2:164) writes that it is logical to assume that since the Torah lists only six metals, only those six are Mekabel Tum'ah. However, even though mid'Oraisa only those six metals are Mekabel Tum'ah, the Gemara relates that the Rabanan instituted Tum'ah for glass, because it has qualities that are similar to metal. Therefore, it is logical to assume that all other forms of metals should at least be considered like glass and be Mekabel Tum'ah mid'Rabanan.
However, RAV MOSHE STERNBUCH shlit'a (TESHUVOS V'HANHAGOS 1:449) suggests that aluminum should not become Tamei even mid'Rabanan. (Accordingly, there should be no requirement, even mid'Rabanan, to immerse aluminum utensils purchased from a Nochri. See Insights to Avodah Zarah 75:1:b:2). The Rabanan decreed that only glass is like metal; there is no basis to assume that any other material is included in their enactment.
Nevertheless, there may be another reason why aluminum can be Mekabel Tum'ah mid'Oraisa. Perhaps the six metals in the verse are mentioned only to exclude other materials which existed at that time (such as glass, which, one might have thought, should have been included in the same category, since it can be molten). The verse does not intend to exclude materials that might be discovered in the future and that have similar characteristics to the six metals. (M. KORNFELD) (See Insights to Avodah Zarah 75:1.)


OPINIONS: The Mishnah and Beraisa teach that small bitter almonds are obligated in Ma'aser, while large bitter almonds are exempt. Large sweet almonds are exempt from Ma'aser, while small sweet almonds are obligated. RASHI (DH Ketanim, DH Mesukim, and DH Ketanim) explains that of the bitter almonds, only the small ones are obligated in Ma'aser, since they are harvested for eating before they grow large and bitter. Sweet almonds, in contrast, are harvested for eating when they are large and fully grown, and thus only large ones are obligated in Ma'aser.
Rebbi Yishmael b'Rebbi Yosi said in the name of his father, "Zeh v'Zeh Liftor" -- "this and this to exempt," while others quoted him as saying, "Zeh v'Zeh l'Chiyuv" -- "this and this to obligate." The Gemara quotes Rebbi Chanina who ruled like the first version of Rebbi Yishmael's statement.
To what do the words "Zeh v'Zeh" refer?
(a) RASHI (DH Zeh v'Zeh) explains that "Zeh v'Zeh Liftor" means that both types of small almonds, bitter and sweet, are exempt from Ma'aser. Similarly, "Zeh v'Zeh l'Chiyuv" means that both types of large almonds, bitter and sweet, are obligated in Ma'aser. Accordingly, when the Gemara asks, "What use do these large almonds have?" it refers to the large, bitter almonds. The Gemara answers that they may be sweetened through fire and eaten.
(b) TOSFOS (DH Zeh v'Zeh Liftor) points out several difficulties with Rashi's explanation. First, the Beraisa defines the categories as bitter almonds and sweet almonds, not small and large ones. Presumably, the statement of "Zeh v'Zeh" refers to the two categories mentioned in the earlier part of the Beraisa. Second, according to Rashi, when the Gemara asks what use do large almonds have, the Gemara should be more specific and ask "what use do large, bitter almonds have?"
Because of these questions, TOSFOS takes a different approach. He says that "Zeh v'Zeh Liftor" means that both large and small bitter almonds are exempt, while "Zeh v'Zeh l'Chiyuv" means that both large and small sweet almonds are obligated. Since there is only one type of large almond (sweet) that is obligated according to the opinion of "Zeh v'Zeh l'Chiyuv," the Gemara's question (what use do large almonds have) can refer only to the large, sweet almond.
It is interesting to note that RASHI in Eruvin (DH Zeh v'Zeh Liftor) explains the Gemara like Tosfos. (See LEV ARYEH here for why Rashi explains the Gemara here differently.)
The words of the Gemara here are quoted by the BEHAG (see ROSH, Berachos 6:3) when he discusses what blessing one recites when he eats almonds. The Behag understands the Gemara like Tosfos and Rashi in Eruvin. He rules that we should follow Rebbi Chanina who says "Zeh v'Zeh Liftor," which means that all bitter almonds are exempt from Ma'aser. Further, he writes that one should recite the blessing of "Borei Pri ha'Etz" for a small bitter almond (see Rosh there at length, where he discusses whether or not the blessing of "Shehakol" should be recited). The BEIS YOSEF (OC 202:5) explains that although the small almonds are bitter, their desirability is their outer skin, which is not bitter when they are small. However, when one eats a large bitter almond, he recites no blessing, because it is more bitter than a small almond, and therefore unfit (and unhealthy) for consumption. This is also the opinion of RABEINU YONAH and the RASHBA in Berachos (36a). (Y. MONTROSE)