Tevul Yom, Chapter 1, Mishnah 4. This mishnah continues a discussion about items stuck onto a loaf of bread and whether they are considered joined to the loaf for purposes of tumah. When something is attached to bread and a tamei person touches that attached item, everything depends on how we classify the item: if it is part of the loaf, the touch has reached the bread; if it is a separate object, the bread is untouched. Here the mishnah lists the cases in which the item stays a thing apart.
"Hatzror shebakikar": a small stone lodged within the bread or fixed onto its surface. "Ve'gargir melach gadol": a sizable grain of salt. "Ve'haturmos": the lupine, that bitter bean, adhering to the crust from without. "Ve'hachirchur yaseir mike'etzba": a charred piece of crust thicker than a fingerbreadth.
Rabbi Yose supplies his own criterion for the crust: "kol she'eino ne'echal imo", anything that is not eaten together with the loaf. For him the fingerbreadth measurement is not the point. What matters is that the burnt portion is so thick that a person would not eat it along with his bread, and that is what leaves it outside the loaf.
Of all these the mishnah rules "tehorim be'av hatumah": they are not counted as part of the loaf, so that even if an av hatumah touches the pebble, the large clump of salt, the lupine, or the thick burnt crust, the loaf itself remains tahor. Only the attached item picks up the tumah; the bread does not.
The closing words, "ve'ein tzarich lomar b'tevul yom", mean that the case of a tevul yom requires no statement at all. A tevul yom carries a weaker grade of tumah than that of an av hatumah. If even contact by an av hatumah with these items leaves the bread tahor, then contact by a tevul yom certainly cannot convey tumah to the bread.
The lesson of this mishnah is therefore the reverse side of the coin: a pebble, an oversized clump of salt, a bitter lupine and a crust too thick to be eaten with the bread are all items a person does not consume together with his loaf, and for that reason halachah treats them as distinct objects resting on the bread rather than as part of it.