Eruvin 86B

קורה ארבעה מתרת במים.

A cross beam of 4 handbreadths laid across a cistern located between two courtyards permits one to draw water from that cistern.

הא קא אזיל דלי לאידך גיסא, ומייתי? קים להו לרבנן דאין דלי מהלך יותר מארבעה טפחים.

With this in mind, the following difficulty arises: The bucket he uses to draw the water might drift under the cross beam to the other side of the cistern and bring water from the other courtyard. The Talmud answers: The rabbis have established that a bucket does not drift more than 4 handbreadths from the point where it was lowered, and it will therefore stay on its original side of the partition.

תחת קורה מיהא הא עריבי מיא! אלא משום דקל הוא שהקילו חכמים במים. כדבעא מיניה רבי טבלא מרב: מחיצה תלויה, מהו שתתיר בחורבה? אמר ליה: אין מחיצה תלויה מתרת אלא במים, קל הוא שהקילו חכמים במים.

The Talmud raises a difficulty: Nonetheless, the water becomes intermingled under the cross beam, and consequently the bucket will bring up water from the other courtyard. Rather, it must be that the reason for the leniency is not that the cross beam actually prevents the flow of the water, but because the rabbis were lenient with regard to water. They allowed a partition suspended above the water to be considered as though it blocked the flow of the water. As Rav Tavla asked of Rav: With regard to a suspended partition, does it permit carrying in a ruin? Do we say that the remnants of the walls suspended in the air are considered as though they descended to the ground and closed off the area, thereby rendering it a private domain? Rav said to him: A suspended partition of this kind permits carrying only in the case of water, as the rabbis were lenient with regard to water.