Eruvin, Chapter Seven, Mishnah 10. This mishnah addresses the question: which foods may be used for an eruv and for a shituf?
The text of the mishnah:
"Bakol me'arvin u'mishtatfin" - any kind of food may be used for an eruv and for shitufei mevo'os, "chutz min hamayim u'min hamelach" - except for water and salt, which are not considered food at all, but rather items outside the category of nourishing food. These are "divrei Rabbi Eliezer" - the words of Rabbi Eliezer.
Which eruv is being discussed?
Many of the commentators explain that the term "me'arvin" here cannot be referring to eruvei chatzeiros, since the halachah is that eruvei chatzeiros are made with bread alone, whereas shitufei mevo'os may be made with other foods as well. It therefore follows that the mishnah is referring to eruvei techumin and to shitufei mevo'os, which may be made with any kind of food except for water and salt.
The view of Rabbi Yehoshua:
Rabbi Yehoshua adds a further criterion, even for an eruv chatzeiros that is made with bread: "Kikar hu eruv" - a whole loaf of bread is required, and a broken slice of bread is not valid. This yields two conclusions:
"Afilu ma'afeh se'ah v'hi perusah" - even if a full se'ah of dough was baked into it, which is a large quantity, if it is broken and is not a whole loaf, one may not make an eruv with it.
"Kikar b'issar v'hu shalem" - even a loaf so small that its price is only an issar, which is a very small coin, since it is completely whole, one may make an eruv with it.
In summary: according to Rabbi Eliezer, an eruv and a shituf may be made with any food, except for water and salt, which are not in the category of food, and this was stated regarding eruvei techumin and shitufei mevo'os. According to Rabbi Yehoshua, the decisive criterion for bread is not the size of the loaf but its wholeness: a broken loaf is invalid even if it is large, and a whole loaf is valid even if it is small.