080: Rabbi Abba: Eiruv Tavshilin must be a k’zayis.
Question: Does a single k’zayis suffice for everyone, or is a k’zayis necessary for each individual?
Rabbi Abba quoting Rav: Eiruv tavshilin must be a k’zayis, whether for an individual or for a hundred people.
Challenge: We learned in the Mishnah (074): “If he ate it [the Eruv] or it was lost, it cannot be relied on. If something was left over, he can rely on it and cook for Shabbos.” So it seems that we can rely on an eiruv as long as ‘something’ is left over - even if it’s less than a k’zayis?
Resolution: ‘Something’ means ‘something that is at least a k’zayis’.
Challenge: We learned: “This cooked item may be broiled, soaked or highly cooked or normally cooked. It may be kulyas ha’ispanin upon which hot water was poured [that is its usual method of cooking]; and it has no measure.” So it seems that there is no measure and a k’zayis is not required.
Resolution: It means that there is no measure in how large it may be, but it must be at least a k’zayis.
Rav Huna quoting Rav: An eiruv tavshilin must be intentional.
Question: The person designating the eiruv must certainly do so with intent; what about the person on whose behalf it is being placed?
Response: Shmuel’s Father would make an eiruv for all of Neharda’a. Rabbi Ami and Rabbi Asi would make an eiruv for all of Tveria. It was the custom of Rabbi Yaakov bar Idi to announce “whoever did not designate an eiruv tavshilin may rely on the one I made”.
Question: Up until how far may people rely on an eiruv?
Rav Nechumi bar Zecharyah in the name of Abayye: Up to a techum Shabbos [since they cannot eat from the eiruv - because they’re not within walking distance, they cannot rely on it].
A certain blind man used to set forth beraisas [he was like a living sefer] before Mar Shmuel. Shmuel noticed that this blind man appeared pensive. Shmuel asked him what was troubling him and he explained that he hadn’t prepared an eiruv. Shmuel told him “you can rely on mine”. When this repeated itself the next year, Shmuel said to him “You’re negligent. Anyone can rely on my eiruv but you can’t”. 
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