1) THE AMOUNT OF WATER NEEDED FOR "TEVILAH" IN A SPRING
QUESTION: The Gemara teaches that when a porous mat (such as a screen) is placed upright in a Mikvah, separating it into two parts, the Tevilah performed in each side is invalid if there is less than forty Se'ah of water in that side. Even though the water flows freely through the mat, the Mikvah is considered two separate Mikva'os since the holes in the mat are not large enough to attach the two Mikva'os to each other. The Gemara proves this from the fact that the ground itself is porous and yet a small Mikvah (with less than forty Se'ah) is not considered attached to any larger water source, even though there is movement of water (terrestrial seepage) through the porous earth between the two bodies of water. Rashi explains that a pool of water springing forth in one place must contain forty Se'ah in order for one to immerse in it. Although the water's source is a large river somewhere else, we do not view the water in the spring to be connected to the river (because the holes of the porous earth are too small).
Rashi implies that the Gemara refers to water that springs forth from the ground. Why, then, does Rashi say that such a body of water requires forty Se'ah? We know that Tevilah in a spring ("Ma'ayan") is valid in any amount of water!
ANSWER: The RASHBA (Toras ha'Bayis, Hilchos Mikva'os) and the ROSH (Hilchos Mikva'os 1:6) prove from the Gemara here that the Halachah that Tevilah in a spring is valid with any amount of water is true only for Tevilah of Kelim. For Tevilah of a person, though, the spring must contain forty Se'ah of water. They infer this from the fact that the Gemara discusses spring-water which seeps through the earth and comes out of the ground, and nevertheless the Gemara implies that there must be forty Se'ah of water in the spring in order for the Tevilah to be valid. This may also be the intention of Rashi here as well.
(See MEROMEI SADEH for further discussion of this Halachah and its ramifications in the Gemara.)
2) IMMERSING A KLI INSIDE OF ANOTHER KLI
QUESTION: The Gemara says that if a person immerses a small Kli, like a needle, inside of another Kli, and the opening in the outer Kli is smaller than "Shefoferes ha'Nod," the Tevilah is invalid and the inner Kli does not become Tahor. However, the Gemara adds that if the outer Kli is also Tamei and thus also needs Tevilah, then just as the outer Kli becomes Tahor through Tevilah, the inner Kli also becomes Tahor.
According to Rava, the reason for the Mishnah's statement (20b) that one may not immerse a Kli inside of another Kli for Kodesh is a Gezeirah lest one use an outer Kli that has a very small opening. The Mishnah, therefore, must be referring to when the outer Kli is Tahor, for otherwise the size of the opening will not prevent the inner Kli from becoming Tahor. Why, then, does the first Rashi in the Mishnah say that one may not immerse a Kli inside of another Kli for Kodesh "when both Kelim are Tamei?" (TOSFOS DH Mai)
ANSWER: The MAHARSHA and the TUREI EVEN write that the answer to this question is evident from Rashi's own words in the Gemara. Rashi writes that if the outer Kli is also Tamei "then the inner Kli becomes Tahor for Terumah" -- but not for Kodesh! As far as Kodesh is concerned, even when the outer Kli is Tamei and becomes Tahor through this Tevilah, the inner Kli remains Tamei. Therefore, when the Mishnah says that one may not immerse a Kli inside of another Kli for Kodesh, that applies even when the outer Kli is also Tamei, as Rashi there says.
How does Rashi know that this leniency (for immersing a Kli inside of another Kli with a very small opening) applies only for Terumah? The TUREI EVEN answers that it is logical to assume that the Chachamim made a Gezeirah for Kodesh, invalidating the Tevilah for the inner Kli even if the outer Kli becomes Tahor through this Tevilah. If the Chachamim made a Gezeirah for Kodesh and decreed that even when the outer Kli's opening is large the inner Kli is still Tamei (lest people immerse in a Kli which has a very small opening), then certainly they also made a Gezeirah not to immerse in a narrow-mouthed Kli that is Tamei so that one not immerse inside a narrow-mouthed Kli that is Tahor.
The Turei Even adds that if such a Tevilah (in a narrow-mouthed outer Kli that is Tamei) would work to make the inner Kli Tahor for Kodesh, then why does the Gemara not mention this as a difference between Rav Ila and Rava? The Gemara, which asked what the difference is between Rav Ila and Rava, should have answered that Rava would be lenient if the outer Kli is Tamei whereas Rav Ila would be stringent (because there is still a problem of Chatzitzah)! (Tosfos is bothered by this question.) It must be, as Rashi says, that both Rav Ila and Rava agree that the inner Kli remains Tamei for Kodesh when immersed in a Kli that is Tamei.
The question still remains, though, why Rashi on the Mishnah says that the Halachah of not immersing a Kli inside of another Kli applies specifically when both Kelim are Tamei. The Mishnah's stringency should certainly apply when the outer Kli is Tahor!
The RASHASH (20b) answers that Rashi understood the Mishnah, when it says that one may not immerse Kelim inside of other Kelim for Kodesh but one may immerse them for Terumah, to be referring to any outer Kli, whether it has a small opening or a large one. If the Kli has a small opening, why then are we lenient for Terumah and consider the water inside the Kli to be connected to the Mikvah? It must be that the outer Kli is Tamei and that is why we are lenient for Terumah, and nevertheless we are stringent for Kodesh.