When there's a non-Jew who lives on the same courtyard or alleyway as Jews, and the non-Jew has another exit from his house, out to a valley... which door gets the primary use? Notably, because of that second door, he doesn't negate the Jews' ability to make an eruv chatzerot. Why is his exit to the valley considered his primary used exit? Because it's uniquely his. But what if it opens to a small karpef, enclosure? That doesn't provide the same privacy or exclusivity. [What's What: MiShmei de-Ulpana] The key value of tradition in the context o eruvin, again! How large does that karpef need to be? Or can it be? And what if the karpef-owner is Jewish? Plus the colorful case of the rock that juts out into the sea. And also a defense of R. Yochanan. Also: Needing warm water at a brit milah, without an eruv to bring some.over from the neighbor - and Rava seems ignorant of the fact that they hadn't set up an eruv. And then it boils down to the Torah law vs. rabbinic law.