Ohalos, Chapter 9, Mishnah 6. This chapter works through the many cases of a kaveres (a beehive, a large earthenware vessel) lying in the doorway of a house, so that part of it is inside and part of it juts out into the airspace outside. The Mishnah keeps changing one detail at a time and asks how tumah travels: into the house, under the kaveres, into the kaveres, or over it.
Our mishnah is the third in a series of parallel cases. Earlier the Mishnah discussed a normal, functioning kaveres whose mouth faces outward, and asked what happens when it is lifted a tefach off the ground. It then discussed a kaveres that no longer serves as a kaveres at all, again raised a tefach. Now the Mishnah returns to a fully functional kaveres, with only one detail changed: its mouth faces inward, into the house, rather than outward. And once again the question is the din when it is not resting flat on the floor but held a tefach above it.
The Case
"Haysah gevohah min ha'aretz tefach": the kaveres was a tefach above the ground. That single tefach of empty space beneath it changes everything, because a gap of a tefach under a covering creates an ohel, and an ohel carries tumah from one end of itself to the other.
The Mishnah then lists where the kezayis of tumah might be found: "tumah tachteha", the tumah is underneath the kaveres, anywhere in that space; "o babayis", or the tumah is somewhere in the house; "o besocha", or the tumah is inside the kaveres itself; "o al gabah", or the tumah is resting on top of it. And the ruling for every one of these is the same: "hakol tamei", everything is tamei.
Why Every Case Comes Out the Same
Begin with tumah lying under the kaveres, even in the section that protrudes outside the house. The tefach of space beneath the vessel is an ohel, so the tumah fills that entire space from one end to the other. Since the kaveres extends into the house, that space under it opens into the house, and the tumah passes inside and fills the house. And once the house is filled with tumah, the tumah enters the kaveres as well, because the mouth of this kaveres faces into the house. Nothing stands in its way.
Take the identical setup from the opposite direction: the kezayis lies somewhere in the house rather than below the vessel. Tumah spreads through the whole house; from the house it descends into the tefach of air trapped below the kaveres and fills it as well; and since this vessel has its mouth turned in toward the house, whatever is within the kaveres becomes tamei too.
Place the tumah inside the kaveres, and the story runs the same way. The tumah exits through the opening, which faces the inside of the house, so the house becomes tamei, and from the house the tumah reaches the space beneath the kaveres.
Finally, place the tumah on top of the kaveres, even on the portion lying outside the house. Here too everything becomes tamei. The tumah descends straight down, and the kaveres is a kley, and a kley does not block tumah from passing through. So the tumah reaches the space underneath, and since that space is a tefach high it is an ohel, and from there the tumah moves into the house and from the house into the kaveres. Once again, everything is tamei.
The One Point of Contrast
So what has this mishnah added, given the earlier case in which the mouth pointed out of the house? There the results were not uniform: keilim stored within that kaveres were shielded, and tumah in the house never reached them. That protection, though, hinged entirely on which way the mouth was turned. Pointed outward, with its cracks plugged up with kash (straw), the vessel stood sealed off against the house's tumah. Once the mouth is aimed at the interior of the house, no such barrier exists, and the tumah travels unhindered in every direction.