The Gemara says that if brine is half (or less) water, the water cannot be Mekabel Tum'ah. According to Abaye, the water in the brine remains Tahor even if more water is later added, such that the brine is now mostly water.
However, Rashi here (DH Tzir Tahor, see Rashash) points out that if a tiny bit of Tamei* water is added to brine so that it is now mostly water, then Abaye will admit that the Tamei drop causes the water is Metamei the water in the brine, which is now in the majority and can become Tamei.
How can a single drop be Metamei? Doesn't water have to have a Shiur (Revi'is or something) to be Metamei other items?
Yosef Kaufman, Yerushalayim, Israel
(Please forgive the delay in response. Technical problems prevented the mailing of a number of responses.)
Here it is not a single drop of water that is Metamei, but rather the drop of water combines with the existing half, and the majority is now Metamei. The existing half does have a Shi'ur, so it follows that the majority of water is Metamei.
You are explaining how the water that was in the brine can become Tamei (Mitamei). Now I realize that some Rishonim require a Shiur for becoming Tamei, but that was not my question.
I am asking how the Tamei drop of fresh water that fell into the brine is able to be Metamei (make the rest of the water in the brine Tamei) after falling in. We know that food cannot be Metamei other things if the food is not a k'Beitzah, and the same should be true with water that is not a Revi'is. The drop certainly does not have a Shiur to be Metamei others and the drop is the lone agent of Tum'ah.
Rashi writes that according to the opinion that Tum'ah which became Batel cannot become Tamei again (i.e. Abaye, as you wrote), one must say that afterwards a drop of the water of the Am ha'Aretz fell in.
Since the water of the Am ha'Aretz is Tamei, it follows that Abaye will agree that the water in the brine returns now to become Tamei, as Abaye said above (23a) that "Tum'ah Me'oreres Tum'ah" -- a drop of Tum'ah can combine with the half of the water that became Tahor in the interim, to return to become Tamei again, since the Tum'ah is now in the majority.
Again, it is not the drop that makes the mixture Tamei, but rather the 50% of the mixture -- which does possess a sufficient Shi'ur to make others Tamei -- now re-awakens with the help of the Tamei water of the Am ha'Aretz, and we have a majority of Tamei water which also possesses the required Shi'ur.