Tevul Yom, Chapter 2, Mishnah 6. This chapter is occupied with the question of chibur, connection. A t'vul yom (one who has immersed but must still wait for nightfall to become fully tahor) renders terumah and kodesh pasul when he touches them, so everything turns on defining what exactly he touched: when several portions of food sit together, do they form one single item, so that contact with one part disqualifies the whole, or is each portion its own entity? Our mishnah applies this to a pot of cooked legumes and to oil resting on wine.
"Ma'aseh kedeirah bekitniyos," a legume dish that was cooked up in a pot. The mishnah now distinguishes two states. First, "bizman shehein perudin": while the individual beans still lie apart, each one keeping to itself, "einan chibur," there is no connection among them. Contact with a single bean therefore stops there and does not travel onward to its neighbors in the pot.
"Bizman shehein gush, chibur." But if they have set together into a single solid mass, they are considered connected, and touching one portion of that mass renders the entire mass pasul.
"Im hayu gushim harbei, harei eilu yimanu." If there were several such masses in the pot, they are counted as separate entities. Each mass stands by itself, and therefore touching one of them does not render the next one pasul.
A second case follows. "Shemen shehu tzaf," oil that floats, "al gabei hayayin," on top of wine. Here the ruling is: "venaga t'vul yom bashemen, lo pasal ela hashemen." Should the t'vul yom put his hand to the layer of oil, the oil alone becomes pasul, while the wine lying underneath is untouched. Why? Because nobody wants these two blended together. Each liquid is meant to keep to itself, and since they are not destined to merge, halachah does not regard them as one food.
"Rebbi Yochanan ben Nuri omer: shneihem chibur zeh lazeh." Rebbi Yochanan ben Nuri rules otherwise. In his opinion the oil and the wine do join into a single unit, so that a touch on the floating oil carries downward and leaves the wine pasul as well.
Both halves of the mishnah teach the same measure: connection is a matter of whether foods have truly become one body. Loose legumes remain individuals, a congealed mass is a single item, several masses are several items, and oil that is meant to stay apart from the wine beneath it is judged on its own.