Keilim, Chapter 8, Mishnah 7. This mishnah turns our attention to the various kinds of earthenware ovens and stoves, and specifically to the small vent that they were built with. It is called the "eye" of the oven, and its function was practical: while the coals were burning, the smoke had somewhere to go. Once the dough was placed inside, the vent would be stopped up so that the heat would build and hold, and the bread would bake properly. And when the heat had to be brought down, or when smoke needed to be let out, that same vent would be opened again.
The question of the mishnah is what happens when a source of tumah is found sitting inside that vent hole, and not inside the baking chamber itself.
"Hasheretz shenimtza" (a sheretz that was found) "be'ayin shel tannur" (in the eye of a tannur), "be'ayin shel kirah" (in the eye of a kirah), "be'ayin shel kupach" (in the eye of a kupach).
The case before us: a dead creeping creature has come to rest inside the eye of an earthenware baking oven, or inside that same little opening in a kirah, which is an earthenware stove, or in a kupach, a narrow earthenware stove built to hold a single pot.
"Min hasafah penimit velachutz, tahor."
Should the sheretz be situated anywhere from the inner rim of the opening outward, the utensil keeps its purity. What this means practically: no part of the creature juts into the hollow cavity where the baking or the cooking is done, and it makes no contact with the inner walls upon which the dough or the pot would sit. It rests within the opening itself and goes no further than that. Since it has neither entered the cavity nor touched any interior surface, the utensil is unaffected and stays tahor.
"Ve'im haya ba'avir, afilu kezayit min hamet, tahor."
The mishnah now moves up to a far more severe tumah. Picture the utensil standing outdoors, with nothing spread above it as a covering. Even if the matter lodged in the eye happens to be a kezayit of flesh from a met rather than a sheretz, the utensil is nonetheless tahor. Two factors work together here: the flesh of the met has not entered the inner cavity and touches none of its interior surfaces, and, a consideration that arises specifically with corpse tumah, no roof spans the utensil and the met at once.
This second factor deserves to be underlined. Where tumat met is concerned, the verdict of tahor depends entirely on the utensil standing in the open. Move that very same tannur into a house or a room, and everything sheltered beneath that roof, the tannur included, would contract tumah, and it would make no difference at all whether the kezayit was lodged in the little opening or lying somewhere else on the floor. A sheretz behaves differently, since it does not convey tumah by way of an ohel; with a sheretz, then, the utensil keeps its purity even under a roof.
"Ve'im yesh sham poteach tefach, hakol tamei."
If, however, the eye is itself a cavity measuring a handbreadth in each direction, a cubic tefach of space, which is the smallest volume that qualifies as an ohel in its own right with respect to tumat met, then the ruling reverses and everything is tamei: the utensil itself, together with whatever is capable of contracting tumah from it.
Why should this be, when the oven is out in the open air and shares no roof with the corpse? Because a corpse fills its tent with tumah. Once the vent hole is large enough to be its own little tent, the tumah fills that hole, and from there it passes into the interior of the oven, which stands open to the vent. And although the oven has no covering above it and lies under no roof, the walls of the oven, which slope slightly inward, are themselves treated as a form of roof, and they draw the tumah down into the oven.
So the picture that emerges is this. A sheretz or a kezayit from a met resting within the vent hole, and no further, leaves the vessel pure. But when that vent hole is wide enough to be considered a tent in its own right, a tefach by a tefach, the hole forms a covering over the piece of the met lying in it, the tumah fills the space, spreads into the oven, and everything becomes tamei.