The Mishna (17a) taught that if the roof of a house fell in, the empty area can be filled with sekhakha – roofing – and will be a kosher sukka if the distance between the walls of the house and the sekhakha is less than four amot (based on the concept of dofen akuma as discussed on yesterday’s daf).
Our Gemara relates that when this halakha was presented by Rabbi Yehuda bar Elai, he simply taught “a house that was breached and one roofed over it is a fit sukka.” Upon hearing this teaching, Rabbi Yishmael the son of Rabbi Yosei demanded clarification. His father had taught that this is only true if the distance from the walls to the sekhakha is less than four amot. If there are more than four amot between them, the sukka will be invalid.
This story is followed in the Gemara by a second, similar one.
Rabbi Yehuda bar Elai taught: With regard to the abramis [avroma], it is permitted to eat it, despite the fact that it is a very small fish that is typically caught in a net with many similar, non-kosher, fish, and it is difficult to distinguish between them. Rabbi Yishmael, son of Rabbi Yose, said to him: My teacher, explain your opinion. Rabbi Yehuda bar Elai said that this is how my father explained it: The abramis found in the rivers of place so-and-so, where there are also non-kosher fish, is prohibited; however, the abramis of a different place so-and-so, where there are no non-kosher fish, is permitted.
Rashi explains that in some places, sheratzim – small non-kosher worms or other creatures – thrive and they cannot be separated from the fish, but in other places there are no such sheratzim.
Rabbi Ya’akov Ettlinger, in his Arukh la-Ner, asks why, in fact, Rabbi Yehuda did not fully explain his statements. He suggests that with regard to the avroma, Rabbi Yehuda may simply have been relating the situation in the place where he lived, where sheratzim were not found. With regard to the case of sukka, this may be connected with Rabbi Yehuda’s opinion that a normal house is up to eight amot in size. Thus, in order for a sukka that is large enough to be the appropriate size and yet fit under the fallen roof, there cannot possibly be more than four amot between the walls of the house and the sekhakha.