One of the laws of the Sabbath and holidays is a limitation on the freedom to move from one’s residence, what Halacha calls “the prohibition of boundaries.” For example, a person who lives in a village or a city may not go farther out of his locality than a kilometer (200 amah), nor may he takes his belongings farther out. Thus, a person may only carry belongings up to one kilometer outside his residential area. If a person finds an abandoned object on a holiday, he may carry this object only one kilometer from the location where it was found, the location which the sages called its “official location.” If rain fell before the holiday and filled an abandoned hole which belongs to none, one may take from these waters and carry them up to one kilometer from the hole. But if the rain falls on the holiday itself, so that on the eve of the holiday their location was unspecified and not subject to the laws of boundaries, it may be carried anywhere, for the rain is not limited by boundaries. The scholars asked: Is it not the case that rain, before it is created in the clouds, on the eve of the holiday was in the ocean, and if it is so, their official location should be the ocean and it should be forbidden to carry them once they have fallen onto land on a holiday, for they have already gone past the legal bounds of their place. The scholars try to explain that this halacha may match the opinion of R’ Joshua, who argued that rain water comes from the upper waters found in the heavens above, so they had no defined place from which to set the boundary. In contrast, another sage, R’ Eliezer, argued that rain water which falls from the clouds is ocean water. One of the scholars , R’ Isaac, argued that the rule above could make sense even based on R’ Eliezer, and rain water which fell on the holiday may be carried anywhere, for this rule deals with clouds which have been marked and are demarcated before the holiday, making it clear the water was not in the ocean but in the clouds, so they did not have a specific location and boundary at the start of the holiday. The scholars asked: Perhaps, because of the movement of the clouds, the clouds were switched around and those which were seen on the eve of the holiday moved on and the rain which fell came after them, their water having been in the ocean on the eve of the holiday? Answer: The clouds had markings so that one could not become confused with other clouds. Another answer: The law is lenient on boundaries when there is room for doubt. The scholars went on to ask: Why is the boundary for rain water not set within the clouds themselves? Answer: Since the water is subsumed within the cloud and is not actually in a liquid state, they cannot create a boundary for themselves. The scholars went on to ask: If so, if the rain water in the clouds changes its state on the holiday from vapor to liquid, one should be forbidden to carry it for a different reason — it was created on the holiday and anything created on a holiday one is forbidden to carry. Based on this question the scholars rejected their earlier opinion and ruled that rain water in the clouds is indeed in liquid form, just as it is when it falls, and the reason their location is not set in the clouds is because the clouds continually move. Any object which moves does not set boundaries for itself. For this reason any rain water which falls on the holiday, even from clouds which formed on the eve of the holiday and whose waters were in the ocean, are permitted for carrying, for ocean waters move and flow, and any moving object does not set boundaries for itself.
(Babylonian Talmud, Tractate Eiruvin 45b-46a)