1) SWIMMING IN A PRIVATE POOL OR BATH ON SHABBOS
OPINIONS: The Gemara says that it is forbidden to be "Oker" (lift up one's feet, -Rashi) in a pool, even if the pool is in a Reshus ha'Yachid, if the pool does not have "Gedudi" (a rim around it). Why is this prohibited?
(a) RASHI explains that "Oker" refers to lifting one's feet off of the floor of the pool, which is prohibited because it constitutes swimming -- an activity that the Rabanan forbade on Shabbos. Thus, if the pool does not have "Gedudi," one may go into the pool on condition that he not lift his feet from the floor.
Regarding the definition of "Gedudi," in his first explanation Rashi explains that "Gedudi" refers to a pool that has high edges. If the pool does not have high edges, it must be that the bottom of the pool rises near the edge. In such a pool, there is concern that one will dig his toes into the dirt on the bottom, causing the dirt and water to be kneaded together. (According to this explanation, it seems that even if one does not lift his feet from the floor of the pool, he is not permitted to use such a pool on Shabbos, since the problem is one of "kneading.")
(b) According to Rashi's second explanation, "Gedudi" refers to a rim around the edges of the pool that gives the pool the appearance of a large vessel. No one will confuse swimming in a vessel with swimming in a river (which is forbidden, lest one make a flotation device). If the pool has no rim, it resembles a river, and therefore one may go in only if he does not actually swim or float (that is, he keeps his feet on the floor of the pool).
(c) The RIF explains that "Oker" means to uproot water from the pool. If the pool has no rim (and is level with the rest of the Chatzer), one may not splash water out of the pool. The water that splashes out of the pool will look like a rivulet and give the impression that one is permitted to swim in a river -- an act which the Rabanan prohibited on Shabbos. However, if the pool has a rim, the rim surrounds the pool-water and prevents the water from splashing out of the pool. It does not look like the pool is part of a rivulet; it clearly appears as an independent vessel. Therefore, one is permitted to swim normally, because it does not look like one is swimming in a river.
(Apparently, Rebbi Zeira's doubt was whether splashing is permitted even when there is a rim, and the conclusion is that it is indeed permitted. It is not clear why splashing should not have been permitted. Alternatively, the RIF permits swimming in a pool with a rim since the rim "surrounds" its waters ("Mehadri Lehu") and makes the water of the pool distinct from the water that splashes out from it. This is why it cannot be confused with a rivulet. If this is the Rif's intention, it is more easily understood why Rebbi Zeira entertained the possibility that it is prohibited to splash in a pool with a rim; he thought that perhaps the rim does not set the pool clearly apart from the water that splashes from it. It is also apparent from the Gemara's conclusion, according to the Rif, that one may swim in a pool without a rim which stands in a Chatzer, as long as he is careful not to splash. However, this Halachah is not cited explicitly by the Poskim. -M. KORNFELD.)
HALACHAH: The SHULCHAN ARUCH (OC 339:2) rules like the Rif (c).
2) PUTTING COLD WATER IN A HOT VESSEL WITHOUT INTENTION TO HARDEN IT
QUESTION: Rav Ada bar Masna explains that the Mishnah (41a) means that when the hot kettle has been emptied of its water, one is permitted to pour cold water into the kettle in order to warm it, even though pouring cold water into a hot vessel tends to harden the vessel. Since one does not intend to harden the vessel, it is a Davar she'Eino Miskaven and is permitted (according to Rebbi Shimon).
However, even Rebbi Shimon agrees that unintentionally performing a Melachah is forbidden when the Melachah will inevitably occur as a result of one's action ("Pesik Reishei"; see Insights to Shabbos 29:2)! Why, then, is one permitted to pour cold water into the hot kettle in order to warm the water, when the vessel will definitely be hardened as a result of pouring in the water?
(a) TOSFOS (DH Meicham) says that it is not certain that the utensil will become hardened.
(b) The RA'AVAD (cited by the Ramban in Milchamos Hash-m) answers that the Gemara does not know at this point that Rebbi Shimon agrees that in cases of Pesik Reishei, even a Davar she'Eino Miskaven is forbidden. The Gemara assumes that he permits a Davar she'Eino Miskaven in all cases, even when it is a Pesik Reishei. The Ramban rejects this explanation.
(c) The RITVA and RAMBAN explain that the Gemara is discussing a utensil that already underwent an extensive hardening process (and it is doubtful whether it can be hardened further).
(d) Alternatively, they suggest that it was doubtful whether the heat at which the utensil was heated was enough to enable it to be hardened when cold water is poured into it.
3) HARDENING A METAL UTENSIL
QUESTION: RASHI (DH Ela Shi'ur) says that a utensil is hardened (through heating it to a very high temperature and then pouring cold water into it) only when it is completely filled with cold water. According to Rashi's words, when Rav Ada bar Masna (41a) says that one is permitted to pour a large quantity of cold water into a hot vessel in order to warm the water, and the Gemara asks that it should be forbidden because of hardening, the Gemara should answer simply that Rav Ada is discussing a case where he did not completely fill the utensil!
Furthermore, Abaye states that if one emptied a hot vessel, one may not put "any cold water in it at all" because of hardening. Why does he prohibit putting any cold water in, if it will be hardened only when it is completely filled?
(a) TOSFOS YESHANIM says that according to Rashi, the words "Kol Ikar" ("any [cold water] at all") are a mistake and do not belong in the Gemara. (See also TOSFOS YESHANIM at the end of the page, who cites MAHARASH M'MIRZBURG who understands the Gemara differently from Rashi.)
(b) Rashi may have learned that the Gemara here, which discusses the view of Rav, does not agree with the Gemara earlier that assumes that hardening can be accomplished with even a small amount of cold water.