Tevul Yom, Chapter 1, Mishnah 3. This mishnah, together with the one that follows it, takes up a single practical question: when some small item is stuck onto a loaf of bread, is that item treated as part of the loaf or as a separate object standing on its own? The question has real consequences for tumah. If a tevul yom touches the attached item, do we say that his touch has reached the loaf itself and rendered it tamei, or do we say that the item is a thing apart, so that whatever happens to it does not travel into the bread?
The mishnah opens with a list of cases in which the added item is considered one with the loaf. First, "masmer she'ahar hakikar": a nail-shaped protrusion attached to the loaf. This is not a metal nail, but a piece shaped like a nail, itself made of dough, that the baker fastened onto the loaf. Next, "ve'gargir melach katon", a small grain or clump of salt sitting on the bread. Next, "ve'hachirchur pachos mike'etzba", a burnt crust of less than a fingerbreadth.
Rabbi Yose offers a different measure for the crust: "kol shehu ne'echal imo", anything that is eaten together with the loaf. In his view the deciding factor is not whether the burnt portion measures a fingerbreadth or less, but whether a person would eat that crust along with the bread. If he would, it belongs to the loaf.
In all of these cases the mishnah rules "teme'im b'tevul yom": the added item is regarded as attached to the loaf, so that if a tevul yom touches the nail-shaped piece, the small clump of salt, or the small burnt crust, the loaf of bread itself becomes tamei. The touch does not stop at the protrusion; it counts as a touch of the bread.
The mishnah continues with the words "ve'ein tzarich lomar bechol hatumos": for the remaining grades of tumah, no separate ruling is necessary. Among all who bear tumah, a tevul yom stands at the weakest end of the scale. So once the mishnah has established that even his contact with these small additions passes through to the loaf, the same conclusion follows automatically for stronger sources of tumah: whoever else touches the nail-shaped piece, the salt, or the crust has thereby made the bread tamei.
The principle emerging from this mishnah, then, is that a small addition which a person treats as part of what he is eating, whether dough shaped into a protrusion, a small bit of salt, or a lightly burnt crust, is halachically one body with the loaf, and tumah that reaches it reaches the bread.