Question (Rav bar Sheva - Mishnah): [If Yom Tov is before Shabbos,] how can one be Me'arev [for both days without risking lest the Eruv get lost]? He takes the Eruv [to the desired place of Shevisah] on Erev Yom Tov, stays there until nightfall, and takes it home with him. The next day he takes it back and stays there until nightfall. He may then eat it and return.

[Why must he bring the Eruv?] We should say Ho'il (since) he could take it with him, it is as if he took it!

Answer (R. Zeira): We decree that he must take it due to when Yom Tov follows Shabbos. (Then, the Eruv is valid only if it is there. Ho'il does not apply, since he may not carry it on Shabbos.)

Question (Beraisa): If one intended to be Shoves in Reshus ha'Rabim and put his Eruv in a wall [outside his four Amos]:

If it is below 10 Tefachim, it is valid. If it is above 10, it is invalid. (He cannot access it Bein ha'Shemashos);

If he intended to be Shoves on top of a dovecote or cabinet [and put the Eruv in a wall nearby - this is like Rashi's preferred explanation (v'I Ika)]:

If the Eruv is above 10 (he and his Eruv are both in Reshus ha'Yachid), it is valid. If it is below 10, it is invalid.

We should say that even below 10 it is valid, for he could tilt the cabinet to below 10. (It would be a Karmelis. There would be no Isur Torah to take it!)

Answer #1 (R. Yirmeyah): The Beraisa discusses a cabinet nailed into the wall. (One cannot tilt it.)

Answer #2 (Rava): Even if it is not nailed in, if it is so tall that if he would tilt it below 10, the top (his Shevisah) would be four Amos away from his Eruv [the Eruv is invalid. It would be an Isur Torah to carry it through Reshus ha'Rabim to bring it. Alternatively, his Eruv would not be in his place of Shevisah.]

Question: If there is a window [in the cabinet next to the Eruv] and he holds a rope (the other end is tied to the Eruv), he could use it to bring the Eruv!

Answer: The case is, there is no window or he has no rope.

(Mishnah): If it is in a pit, even 100 Amos deep [it is valid].

Question: Where is the pit?

If it is in a Reshus ha'Yachid, obviously it is valid. Just like Reshus ha'Yachid extends up to the sky, it descends to any depth! (Even if he intended to be Shoves above, he can take his Eruv.)

Answer #1: It is in a Reshus ha'Rabim.

Question: Where did he intend to be Shoves?

If he intended to be Shoves above, he and his Eruv are in different places!

If he intended to be Shoves in the pit, obviously it is valid. He and his Eruv are in the same place!

Answer #2: It is in Karmelis, and he intended to be Shoves above. Our Mishnah is like Rebbi, who permits Shevus during Bein ha'Shemashos. (Therefore, he may take his Eruv from the pit.)

(Mishnah): If he put the Eruv on top of a reed or pole, if it was initially detached and later inserted into the ground, it is valid.

(Gemara - Rav Ada bar Masnah) Question: The Eruv is valid only if it was detached and later inserted. If it was attached, it would be invalid!

This is like Chachamim, who forbid Shevus (e.g. using something attached) during Bein ha'Shemashos.

We established the Reisha (the previous Mishnah) to be Rebbi. Is the Reisha Rebbi and the Seifa Chachamim?!

Answer #1 (Rava): Rami bar Chama already asked this. Rav Chisda answered that indeed, the Reisha is Rebbi and the Seifa is Chachamim!

Answer #2 (Ravina): Also the Seifa is Rebbi. He decrees to forbid [an attached reed] lest one cut off the top when taking his Eruv. (We are not concerned for this regarding a tree, for it does not break easily.)

The king's army came to Neharda'a. (This caused a lack of room for Talmidim.) Rav Nachman said [to his Talmidim] 'go bend reeds. Tomorrow (Shabbos) we will sit on them.'

Question (Rami bar Chama - Mishnah): If it (the reed) was initially detached and later inserted, the Eruv is valid.

Inference: If the reed is attached, the Eruv is invalid! (This shows that an attached reed is like a tree, and it is forbidden on Shabbos.)

Answer (Rav Nachman): That refers to Uzradin (reeds that became hard like a tree).

Question: What is the source to distinguish Uzradin from regular reeds?

Answer (Beraisa #1): Reeds, thorns and Hegin (a prickly tree) are kinds of trees. They are not Kil'ayim in a vineyard;

Contradiction (Beraisa #2): Reeds, Kidan (costus, an ingredient of the Ketores) and Urbanin (Rashi - a species of willow; Tosfos - a certain vegetable) are kinds of vegetables. They are Kil'ayim in a vineyard.

Resolution: Beraisa #1 discusses Uzradin. Beraisa #2 discusses regular reeds.

Question: Kidah is not a vegetable!

(Mishnah): One may not graft Pigam (rue, an herb) onto white Kidah, for this is a vegetable on a tree.

Answer (Rav Papa): Kidah and white Kidah (cassia) are different.

(Mishnah): If one was Me'arev in a cabinet and lost the key, even so it is valid;

R. Eliezer says, if he does not know where the key is, it is invalid.

(Gemara) Question: Why is it valid? [Since he cannot access his Eruv, it is as if] he and his Eruv are in different places!

Answer #1 (Rav and Shmuel): The Mishnah discusses a cabinet of arranged bricks [not cemented together]. The first Tana is like R. Meir, who permits opening it:

(Mishnah): If a house full of produce was sealed, and part [of the wall] fell [on Shabbos or Yom Tov], one may take produce through the opening;

R. Meir says, one may pierce [the wall] and take l'Chatchilah.

(Rav Nachman bar Ada): The Mishnah discusses arranged bricks. (One may not pierce a proper wall.)

Likewise, our Mishnah discusses bricks arranged like a cabinet!

Question: R. Zeira taught that this is permitted only on Yom Tov, but not on Shabbos!

Answer: Indeed, also our Mishnah permits only on Yom Tov, but not on Shabbos!