Ohalos, Chapter 8, Mishnah 6. Our mishnah takes up a case that tests exactly how far the protection of a sealed earthenware vessel reaches, and it teaches a striking result: the same seal that saves the vessel does not save the house.
Two Barrels, Two Half-Kezeisim
Our mishnah opens with "Shtei chaviyos": two barrels, both of them earthenware vessels, keilim of cheres. It then tells us what lies within: "uvahen kishnei chatzoi zeisim", inside these barrels are two fragments of a meis, each fragment measuring only half a kezayis, one of them lying in the first barrel and one in the second.
The mishnah continues: "mukafos tzamid pasil". This is a term familiar from Maseches Keilim. Mukaf means encircled, from the same root as hakafos, going around and around; tzamid pasil means a tight, complete seal. In our case both barrels are sealed shut in this manner. And "umunachos besoch habayis", both of them are sitting inside the same house.
The Ruling: The Barrels Are Tahor, the House Is Tamei
The mishnah rules: "hein tehoros", they are tahor. That means the barrels themselves and whatever else is stored inside them remain tahor. After all, if you examine each barrel by itself, it contains less than a kezayis of tumas meis, and less than a kezayis is not enough to be metamei.
But the two barrels stand under one roof, under a single ohel. And so the mishnah concludes: "vehabayis tamei", the house is tamei. As throughout this masechta, "the house" means whatever is found inside the house: any object that is not enclosed in a vessel that is mukaf tzamid pasil, not fully surrounded and sealed. All of that becomes tamei.
How Can It Be Both?
A question immediately demands attention: does a tight seal shut the tumah in or not? And the answer turns out to be yes and no together. The Bartenura draws the line for us. An ohel operates in two directions at once: tumah outside it cannot get in, and tumah underneath it cannot get out. A sealed earthenware keli has a more limited power. Nothing penetrates it from the outside, yet what is inside is not prevented from making its way out.
That single distinction accounts for the whole ruling. Because nothing comes in, the barrel and its contents are protected and remain tahor. But because the tumah inside is not held back from going out, the two half-kezeisim are treated as though they emerge and meet under the one roof of the house. Two halves under a single ohel join into a full kezayis, and the house is tamei.
When One Barrel Is Opened
The mishnah goes on: "niftecha achas meihen", one of the pair was opened up, while the second is still mukaf tzamid pasil, sealed all around. Now comes the verdict, "hi vehabayis temei'im": the barrel that was opened is tamei, and so is everything the house holds. And then "vehachaverta tehorah": its partner, the one whose seal is intact, is untouched and remains tahor.
Here the very same principle is applied in two directions. With respect to the house, a tzamid pasil seal was never able to detain the tumah within, so once more the two halves are viewed as coming out and joining into a complete kezayis, which renders everything in the house tamei. With respect to the barrels, a tzamid pasil seal does bar tumah from entering, so the barrel that is still sealed is truly tahor. The barrel that was opened enjoys no such shield: it is sitting beneath a roof that spans a meis, and it therefore contracts tumah.
The Parallel Case: Two Side Rooms
The mishnah then extends the halachah: "vechein shenei chadarim shehein pesuchim labayis", and the same law applies to two rooms that open into the house. (The word cheder simply means a room. We call a yeshiva for young children a cheder because there was a dedicated room in which the rebbi learned with them.)
Imagine a house with a chamber on its right side and a chamber on its left, and between them a corridor or central chamber. The front entrance of the house does not lead into either of the flanking chambers; it leads into the space in the middle. From that middle space a doorway leads into each flanking chamber, but neither of them has any opening to the street. Now place half a kezayis of tumas meis in each of them, and shut both of their doors.
What becomes tamei? Only the middle room. Here there is no sealed vessel at all, so what is at work? The Bartenura, in his closing words, invokes a principle we have already met: sof tumah latzeis, the tumah is destined to leave. It will not remain in the house forever; eventually it will be carried out. And since the only route out is through the middle area, the tumah is regarded as passing through there. That is enough to make the middle room tamei.
The flanking chambers, however, are unaffected and stay tahor. The Bartenura supplies the reason at the end of his comment: ein derech tumah lehikanes, tumah has no path of entry. One could object that the tumah is sitting right there inside those chambers! But what is sitting there is a mere half kezayis, and half a kezayis by itself lacks the shiur to be metamei. It is only the rule that tumah is headed outward which pulls each half toward the central space and out through the entrance, and it is there that the two halves total a full kezayis, rendering the central space tamei. Inward movement, though, is no part of tumah's nature, and the doors are shut, so each half kezayis stays sealed off where it lies, and neither chamber holds a sufficient measure to be affected.
Summary
We began with two sealed barrels in one house, half a kezayis of tumas meis in each. A sealed earthenware vessel keeps tumah from entering but does not keep tumah from leaving. Since the tumah is not confined, the two halves are treated as emerging and combining into a full kezayis, so everything in the house is tamei, while, remarkably, the barrels and their contents remain tahor.
Should one of the barrels be opened, the house is tamei on the identical grounds; the barrel that was opened contracts tumah, since it now sits unshielded beneath a roof that spans a meis; and the barrel whose seal is intact remains tahor.
Finally, the mishnah applies the identical logic to a house with two side rooms holding half a kezayis each, with no exit to the outside except through the central area. Because derech hatumah latzeis, both halves are considered to pass out through the middle, joining there into a full kezayis and rendering the middle room tamei. But with the doors closed and no principle bringing tumah inward, each side room is left with only half a kezayis, and both remain tahor.