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1. Although the Chachamim say that the Se'ah of Terumah Teme'ah becomes nullified, and Rebbi Eliezer says that it is taken out of the mixture (see Daf 22, #5), they agree that one may not eat the grain in a manner in which it can become Tamei.
2. When a mixture contains a minority of a Tamei item and a majority of a Tahor item, the minority amount is nullified but maintains its identity to a certain degree.
3. Bar Pada: Food that has a high degree of Tum'ah (such as Tum'as Neveilah) must be fit for a person to eat in order to transmit this Tum'ah.
4. Rebbi Yochanan: Food that has a high degree of Tum'ah must be fit for a dog in order to transmit this impurity.
5. Raban Gamliel: If a person buys an animal that is nursing its young, the future offspring of the mother animal is exempt from the laws of a firstborn animal.
A BIT MORE
1. The Gemara explains that this is due to a decree that one might mix a little more than a Kav of this grain with a Tamei Kav of Chulin grain in order to cause the mixture to become Tahor. He will not realize that this mixture has some Tum'ah in it, which causes the entire amount to be Tamei.
2. For example, one who touches a mixture of ashes comprised of a majority of regular ashes and a minority of ashes of the Parah Adumah does not become Tamei, since the ashes of the Parah Adumah are nullified in the majority of regular ashes. However, if he carries the mixture he will become Tamei, since he clearly is carrying the ashes of the Parah Adumah.
3. Bar Pada derives this from the verse, "Do not eat any Neveilah, to a Ger Toshav." The Torah mentions a Ger Toshav to teach that the meat must be fit for a person in order to be considered Neveilah.
4. Rebbi Yochanan understands that the verse, "Do not eat any Neveilah, to a Ger Toshav.," excludes a case of an animal that was rotting away while it was still alive, and was unfit for a person to eat when it died. However, he understands that if it was fit for a person to eat when it died, it is considered Neveilah until it becomes unfit for a dog to eat.
5. We do not suspect that the baby animal that is nursing from it is not its child.
Next Daf Index to Revach for Maseches Bechoros