Tevul Yom, Chapter 2, Mishnah 7. A tevul yom is a person who had contracted tumah, immersed in a mikveh, and is now waiting for nightfall to complete his taharah. During that in-between stretch of the day he has a limited kind of tumah: he invalidates terumah that he touches, but chullin liquids are not affected by his touch at all. Our chapter works out what happens when the tevul yom touches one part of a larger body of food or liquid, and how far his touch is treated as reaching. This mishnah presents an unusual case: a barrel of terumah wine that has gone under the surface of a pit of ordinary wine.
"Chavis sheshak'ah lesoch bor shel yayin": a barrel holding terumah wine sank down into a pit filled with chullin wine. The barrel lies completely beneath the surface, with the pit's ordinary wine standing above it and covering it over. "Venaga bo tevul yom": a tevul yom now puts his hand into that ordinary wine resting above the sunken barrel. He had no contact with the vessel itself, and none with the terumah wine held inside it. What the mishnah must determine is whether his touch is considered to have extended down to the terumah.
The mishnah draws a line based on where in the pit he put his hand. "Min hasafah velifnim, chibur": if the spot he touched lies within the rim of the barrel, meaning straight above the barrel's mouth and inward from its edges, the wine at that spot is regarded as attached to the terumah wine underneath. Since the liquid forms one continuous mass, his contact with the upper wine is reckoned as contact with the terumah, and the entire quantity is thereby disqualified.
"Min hasafah velachutz, eino chibur": if the point he touched lies beyond the line of the barrel's mouth, off to the side, no connection is established. What he handled was chullin wine, and a tevul yom has no effect on chullin liquids. Since that wine does not count as attached to the terumah inside the barrel, no tumah has been transmitted anywhere, and not a drop of the wine is disqualified.
Rebbi Yochanan ben Nuri takes the connection further. "Afilu al gabav rum komah": even where the ordinary wine piled above the barrel reaches the height of a full-grown man, "venaga keneged piha", and the tevul yom's contact was made at a point lined up with the barrel's mouth, "chibur", the two are joined. No matter how deep that column of liquid may be, his touch is carried straight down to the terumah wine beneath it, so the terumah has now received the tumah, and the whole amount of wine is disqualified.
The principle to carry away from this mishnah is that a body of liquid can be treated as a single connected unit even when a barrel wall stands in the middle of it, and that everything turns on the vertical line of the barrel's opening: within the rim there is chibur, outside the rim there is none.