Tevul Yom, Chapter 3, Mishnah 2. Our subject is chibur, halachic joining: at what point do two things count as one single body, so that tumah arriving at one of them spreads through to the other as well?
The mishnah begins with a broad rule, "Kol yados ha'ochlin shehen chibur be'av hatumah." Foods frequently come with a yad, a handle: some appendage joined to the food which a person grips instead of taking hold of the food itself. Wherever that handle is regarded as one with its food in the case of an av hatumah, so that an av hatumah that comes into contact with the handle renders the food itself tamei, the identical rule governs a tevul yom, and this is what the mishnah means by the words "chibur bitevul yom." Should a tevul yom handle the yad, the food joined to it is affected.
A piece of food that cracked but did not fall away
The mishnah now turns to "ochel shenifras ume'oreh miktzas," a piece of food that split and began to come apart into two pieces, yet the two remain to some degree attached to one another. Is this still one food, so that tumah touching one part travels to the other, or have we two separate foods? On this the mishnah records a four way machlokes.
Rebbi Meir omer: "im ochez bagadol vehakatan oleh imo, harei hu kamohu." Take hold of the larger piece. If the smaller piece rises along with it and does not break away, the two are one food. If, however, you lift the larger piece and the smaller one falls off, they are no longer one food, and tumah does not pass from one to the other.
Rebbi Yehuda measures matters from the other end. He says "im ochez bakatan," one grips the small piece, and then "vehagadol oleh imo, harei hu kamohu": should the large piece hold fast and rise together with the small one in your hand, we treat them as one food. Should the large piece tear loose instead, which is of course the far likelier result than a small piece dropping when one lifts by the large, then we have before us two distinct foods, and contact with the small piece will not automatically bring tumah to the large one.
Rebbi Nechemya omer: "batahor." In his view the examination is carried out with whichever side remained clean, the side no contact reached. Lift that piece, and if the side which the tevul yom handled clings to it and stays put, the two are a single food; should that side come loose and drop, they are two.
Vachachamim omrim: "batamei." The Chachamim reverse this. One takes hold specifically of the portion that the tevul yom touched and sees whether the remainder holds together with it or falls away.
All other foods
The mishnah closes with a rule for foods generally: "ushe'ar kol ha'ochlin, es shedarko le'echoz be'aleh, ochezin oso be'aleh, uvekallach, ochezin oso bekallach." A food that people ordinarily pick up by its leaf is tested by holding it by the leaf, and one that is ordinarily picked up by its stalk is tested by holding it by the stalk. Whichever part is the natural one to grasp, that is the part by which we lift the food and watch what happens. If it bears the rest of the food along with it, the two are a single item; if the rest breaks off, we know that they are two separate foods and no longer one for the purposes of transferring tumah.