Eruvin 86 - Water between two courtyards

When someone leaves his home for Shabbat, he still needs to be part of the eruv (common food), since he may return on Shabbat. Rabbi Shimon says that he does not, since it is highly unusual for people to return once they left for Shabbat - with the exception of a parent visiting his or her daughter-in-law, because then if there is a quarrel, it has a tendency to continue.

If there is water, for example, in a cistern, between two courtyards, then when the residents of one courtyard draw water, they are drawing from the other courtyard - because water always mixes - and in the absence of a common eruv, it is prohibited. What is to be done? They must put up a separation, a wall within a cistern. Even an incomplete wall, which only partially shows up and is partially submerged in water, is sufficient - provided that it is ten handbreadths high. There are multiple opinions on the exact position of this separating wall, but all agree that this is a leniency that the Sages gave to the water and that a partial "hanging" separation will not suffice on dry land.

Art: Well Millstone And Cistern Under Trees by Paul Cezanne

Don't understand a point? Ask MosesAI about it.