Eruvin 79: Learning from the 3 Little Pigs

3 mishnayot on things that can come between courtyards and change the need for eruvin: 1. A ditch between two courtyards. The Gemara delves into the authority behind the mishnah's psak, and also attempts a comparison and then rejects it. 2. A haystack that is between 2 courtyards that is 10 tefachim high functions as a partition, until the livestock from either/both courtyards reduce it to lower than 10 tefachim high. 3. Establishing an eruv in an alleyway that runs by courtyards - how to do it? Use a barrel full of food and make it "everyone's." That gives them the common interest cooperative. But how do all those people accept the food in the barrel if they aren't there at that moment? "Zakin le-adam shelo be-fanav." A formal act of acquisition on behalf of someone else, which itself can only be performed by some people, but not all. Including making an eruv for a city. Plus: Pompedita.