Eiruvin 77 - 83
The Whole Story
What is the connection between an eruv and a Shabbos meal? Aside from the fact that both of them are related to the laws of the Sabbath they share a particular law connected to the wholeness of a loaf of bread.
The bread used for eruvei chatzeros to allow the residents of different houses in a courtyard to carry from their individual homes into the common courtyard must be a whole one. Each household must contribute a whole loaf of bread (or contribute flour to the baking of a common complete loaf according to some authorities) even if it is a small one, but not a partial loaf even if it is very large (Shulchan Aruch Orach Chaim 366:6).
At each of the Shabbos meals we are required to "break bread" over two complete loaves (for which challahs are traditionally used). Even in the weekdays it is preferable to make the blessing of "hamotzi" over a complete loaf (ibid. 274:1 and 168:1-2).
What if a loaf designated for one of the aforementioned purposes is no longer whole because a part of it broke off?
Rabbi Chisda suggests an ingenious solution. Take a toothpick and stick it into both parts of the loaf and thus "sew" them together. If this succeeds in creating the appearance of a whole loaf because the "seam" is not readily recognizable, then the loaf is considered whole.
Why is appearance alone sufficient when in reality only a superficial wholeness exists?
Regarding eruvin, the insistence on a whole loaf is based on the consideration our Sages give to neighborly relations. If one household could contribute a partial loaf, his neighbor might resent the fact that he did not give a whole loaf like himself and this might lead to discord within the community. Once the loaf has the appearance of a whole loaf this danger no longer exists.
Regarding the whole loaf for the Shabbos meal, the objective of honoring the holy day with two whole loaves (or the blessing of "hamotzi" on the single whole loaf in the weekdays) is also achieved if the loaf has the dignified appearance of wholeness.
Room For More
How much food constitutes a meal? This question arises regarding the law of eruvei techumin. On Shabbos, a Jew may not walk more than two thousand amos (approximately a kilometer) beyond the physical limits of his community unless he makes an eruv within that distance. This enables him to walk two thousand amos from the location of the eruv.
The eruv is made by placing enough food for two meals at a designated location before Shabbos, thus establishing that spot as his virtual home for that Shabbos.
How much bread (or other food detailed in Orach Chaim 286:5-6) constitutes two meals is a subject of debate in our mishna. Rabbi Meir states that the criterion for an eruv meal is how much one eats in a weekday meal, not how much he consumes in a Shabbos meal. Rabbi Yehuda holds the opposite - how much he eats in a Shabbos meal is what counts.
Both approaches, explains the mishna, are based on the fact that the requirement of an eruv is a rabbinic one rather than a Torah law. The Sages were therefore more lenient in their requirement. Both Rabbi Meir and Rabbi Yehuda aimed to designate an amount of food which would be less demanding, but they approached their undertaking from opposite angles.
On Shabbos, contends Rabbi Meir, the food is tastier than in the weekdays and a person is therefore accustomed to eating more. In order to be lenient, we must therefore establish the amount he eats in the weekdays as the standard for the eruv. Rabbi Yehuda, on the other hand, points out that on Shabbos a Jew eats three sumptuous meals in contrast to the weekdays, and therefore he consumes less in each of those meals than he does in a single weekday meal.
The gemara relates that Rabbi Yosef acted in accordance with the view of Rabbi Meir because of the common wisdom that "the stomach expands to accept tasty food."