Tevul Yom, Chapter 2, Mishnah 5. The theme running through this chapter is the question of chibur, connection. A t'vul yom (one who has immersed but must still wait for nightfall to be fully tahor) renders terumah and kodesh pasul by touching them. So everything depends on defining the boundaries of what he touched: when several foods sit together, are they one single item, so that a touch on one part disqualifies all of it, or are they separate entities, each standing on its own? This mishnah applies that question to a cooked dish of kodesh.
The case: "Besar hakodesh shekaram alav hakipah," meat of kodesh, meat of a korban, over which the kipah has congealed. When such meat is cooked, a layer of sediment and thickened gravy settles and hardens on the surface, forming a kind of crust or cap over the pieces of meat below.
Now the halacha: "Naga t'vul yom bakipah, chatichos mutaros." If the t'vul yom touched only that congealed upper layer, the pieces of meat lying underneath remain permitted. The kipah is treated as its own entity, and it alone becomes pasul; the meat beneath it is not drawn in by the touch.
The reverse situation produces the opposite ruling. "Naga bachaticha," should he make contact with one of the chunks of meat, that chunk, "vechol ha'olim ima," together with whatever rises along with it, is "chiburin zeh lazeh," bound together as a single food. Anything adhering to that chunk and coming up when it is raised counts as part of one and the same item, so the entire mass becomes pasul.
"Rebbi Yochanan ben Nuri omer: shneihen chiburin zeh lazeh." Rebbi Yochanan ben Nuri holds that the two are connected to one another, so the distinction drawn above does not stand; the congealed layer and the meat below it form one item either way.
The mishnah ends by widening the rule beyond meat: "Vechen bekitniyos," and the identical halacha governs legumes, "shekarmu al gabei prusos," which have set and hardened upon slices of bread. Such a dish is viewed as a single joined mass, so that contact by a t'vul yom anywhere in it disqualifies the whole of it.
The lesson of the mishnah is that connection is not decided by proximity alone but by whether the foods have genuinely become one body. A crust resting on top of meat is its own layer; food that clings to a piece and lifts with it, or legumes that have set into the bread beneath them, have become one item, and a single touch reaches all of it.