Tevul Yom, Chapter 3, Mishnah 1. This mishnah takes up a barrel that is a kli cheres, an earthenware vessel. When earthenware was fired in the kiln, it happened not infrequently that a bubble would form in the wall of the vessel: a hollow pocket sitting inside the thickness of the clay.
A bubble of this kind may wind up perforated in two directions. One puncture can open inward, letting whatever liquid is stored in the vessel seep in and fill the pocket, and a second puncture can open outward, giving that liquid a path out of the vessel entirely. What the mishnah comes to determine is this: does the liquid resting in the pocket form a single unit with the liquid held in the barrel, so that tumah reaching it in the pocket carries through to everything within?
The two holes opposite one another
The mishnah opens: "habu'ah shebechavis shenikvah, bein mibifnim bein mibachutz," a bubble in a barrel that was punctured, both on the inner side and on the outer side, "bein milma'alah bein milmatah," whether the punctures are high up or low down, "zeh keneged zeh," so long as the two of them correspond to one another and stand at the same height. In such a case, if a person who is an av hatumah touches the liquid through the outer hole, the ruling is "tamei be'av hatumah vetamei be'ohel hames." The tumah passes on, and the liquid in the barrel becomes tamei. And the same barrel is likewise rendered tamei if it stands under one roof together with a corpse.
The inner hole low and the outer hole high
The mishnah turns next to "hapnimi milmatah vehachitzon milma'alah," a case in which the puncture facing inward is the lower of the two and the puncture facing outward is the higher. In this arrangement as well, an av hatumah who reaches the liquid at the outer puncture is coming into contact with liquid positioned higher than the spot where the pocket takes in from the vessel, and such liquid is considered attached to what lies within. Hence the ruling: "tamei be'av hatumah vetamei be'ohel hames." The tumah conveyed by the av hatumah spreads to everything the barrel holds, and should the barrel find itself beneath a single covering with a corpse, its entire contents contract tumah too.
The inner hole high and the outer hole low
There remains one more arrangement, "hapnimi milma'alah vehachitzon milmatah,": the puncture that leads into the vessel is the upper one, while the puncture that leads out is the lower one. Now an av hatumah reaching for the liquid at the outer puncture makes contact at a spot situated below the place where the pocket joins the vessel. The ruling accordingly shifts: "tahor be'av hatumah." That contact does not render what the barrel holds tamei. Still, the mishnah adds "vetamei be'ohel hames." Tumas ohel does not operate by having one body of liquid transmit tumah to another, and therefore such a barrel, when it is covered together with a corpse, is tamei no differently from any other.
The three cases together give us a clear measure. Everything turns on where the outer hole sits relative to the inner one. When the point that the tamei person touches is level with the opening into the barrel or above it, the liquid there is one with the liquid inside and the tumah travels. When it lies below that opening, the touch remains outside, and the barrel stays tahor.