Explanation #1 (Rashi): The Seifa is the Mishnah (101a) [that one in Reshus ha'Yachid] may not unlock [in Reshus ha'Rabim], in which R. Meir teaches.

Question: That is too far away! (There are five Mishnayos in between.)

Explanation #2 (Ri): The Seifa is "one may not [stand in Reshus ha'Yachid and] urinate or spit [to Reshus ha'Rabim]", with which R. Yehudah argues. The Stam Tana who argues with R. Yehudah is R. Meir.

Question: If so, the Gemara should have asked also a contradiction in R. Meir, according to the text in most Seforim "R. Meir is Metamei", and so is R. Chananel's text.

Observation: According to the conclusion that the opinions must be switched, it is fine that there is no contradiction in R. Meir.

Answer: Perhaps this is why it did not ask the contradiction in R. Meir, because some switch R. Meir's words to R. Yosi.

Explanation (Ri): The text does not say "v'Nach" (and it rested), for v'Nach connotes that he did not intend for this, like is proven in Shabbos (5a) regarding R. Yochanan's teaching that if one threw an item and it went v'Nach in his friend's hand, he is liable;

Here we discuss when he intended [that it rest in the dog's mouth]!

Remark: However, R. Chananel's text says "v'Nach". We can say that v'Nach properly connotes that he intended, for Stam, he threw and the item landed where he intended.

Distinction: However, "it went and was Nach" connotes that it went without the person's intent.

Alternatively, in Shabbos (5a), from the superfluous teaching we infer that he discusses without intent.

Explanation: [His intent makes it considered a place] only in such a case, that he benefits in that place more than elsewhere, e.g. in a dog's mouth, that it will eat it, or in the opening of the furnace, that it will burn there, and similarly when he urinated or spat;

However, if he threw on sticks or on anything, even with intent, we do not say that his intent makes it a place.

Explanation #1 (Rashi): We discuss a fig cake of Terumah.

Explanation #2 (Ri): It is fine even if it is of Chulin. Hands that disqualify Terumah, they are Metamei saliva, which is a liquid, to be a Rishon, and it returns and it is Metamei the fig cake, and it is also Machshir it. It is Tamei Chulin, which disqualifies [one who eats it from eating Terumah];

Any amount of a [Tamei] liquid is Metamei, like we say in Brachos (52a) "lest water come on back of the cup and it will be Metamei the hands";

Normally, there is not a Revi'is on back of the cup, for in his entire Netilah is only a Revi'is (i.e. Stam, his cup holds only a Revi'is. Perhaps the text should say "Natla", a Kli that holds a Revi'is - Chulin 107a, Rashi Gitin 69a.)

Support: Also Pesachim (14b) connotes that the meat is Tamei due to the liquid on it, and presumably there is not a Revi'is.

Implied question: It says in several places "to be Metamei Tum'ah of liquids, a Revi'is [is required]" regarding wine, and [it says so] regarding 10 [matters for which the Shi'ur is] a Revi'is!

Answer: That is to disqualify the person, like the Mishnah in Me'ilah (17b) teaches "all liquids join to a Revi'is to disqualify the person", but to become Tamei and be Metamei food, any amount of liquids become Tamei and are Metamei;

Saliva is considered a liquid to receive Tum'ah, just like it is considered a liquid to be Machshir the fig cake.

Support: Now, what it says in Chagigah (20b) "a string snapped for me, and I tied it in my mouth", according to what I explained, it is fine even if [she was Tehorah, for] even saliva of a Tahor person [can become Tamei through his hand, and be Metamei other matters].

Also R. Yom Tov explained "a string snapped for me, and I tied it in my mouth", and at the time that there was woven enough to receive Tum'ah, there was still moist saliva on the thread, and it became Tamei due to the hands, and it was Metamei the cloth [that she wove], because any amount of a Tamei liquid is Metamei, like I explained.

This is unlike R. Tam explained, that it discusses saliva of a Nidah, like Rashi explained there, because he did not want to establish it to discuss saliva of a Tahor, because saliva is not considered a liquid to receive Tum'ah.

Rebuttal: It is properly considered a liquid, like I explained.

Alternative text: Some texts say "R. Meir is Metaher."

Explanation: "Regarding Ma'aser" means that he must enter his head and majority over the winepress, and then he is permitted to drink in every case. This is like R. Meir.

Remark: This is Rashi's text. I.e. [one may collect] from the air, but he may not put his mouth or a Kli to the opening of the gutterpipe within three of the roof;

Even though [his mouth or the Kli] is below 10, he is like one who is Motzi from the roof to Reshus ha'Rabim, since it is within three of the roof.

Explanation: He may even be Metzaref (put his mouth close), since it is below 10 and it is not within three of the roof, so Lavud does not apply.

The Gemara establishes when it is not four [Tefachim]. If it were four, Tziruf would be forbidden, for he takes from Karmelis to Reshus ha'Rabim.

Support: The Tosefta proves like Rashi's text. It permits Tziruf to a pipe below 10. Therefore, it seems that Rashi's text is correct. Regarding a gutterpipe it says below 10;

If the text were above 10, it would connote that a pipe is permitted even above, and below, even a gutterpipe is permitted;

The Tosefta permits only a pipe below, but a pipe above 10, or a gutterpipe even below, is forbidden.

Alternative text: Some texts say "from the pipe", and in any case it is permitted;

Rebuttal: Rashi challenged this text in the Gemara.

Alternative text: This is R. Chananel's text. Rashi's text says "one may not stand in Reshus ha'Yachid..." It means as follows. One may not stand on the roof and be Metzaref from his friend's roof that is 10 Tefachim taller, and they were not Me'arev;

This is even like R. Meir, who says that roofs are one Reshus, for when a roof is 10 higher, he agrees.

This can be even according to R. Shimon, for we can explain that Tziruf is forbidden, i.e. in order to bring [the water] to the house. Collecting is permitted, even to bring it in, e.g. if [the water] was not Shoves on his friend's roof;

If it was Shoves, obviously he may not bring it to the house!

In any case, even though it was not Shoves, he may not be Metzaref and bring it to the house, for this is truly from the roof to the house.

Disclaimer: The words of the Beraisa do not connote like R. Shimon. It says "but one may collect and drink" connotes in his place. This connotes that Tziruf is forbidden even in his place!

Implied question: Why does it say in Gitin (79b) that Rav Yehudah said in the name of Shmuel that one may not stand on his roof and collect rainwater from his friend's roof, i.e. to bring it to the house, like I explained above?

Answer #1: We must say that this is when it was Shoves on the roof. If it was not Shoves, it is permitted, like we say here.

Answer #2 (Mahari): There we discuss to collect from the air [above] his friend's roof. Here he collects from the air [above] his roof, close to his friend's roof.