Keilim, Chapter 10, Mishnah 3. This chapter is occupied with the tzamid patil, the covering that is sealed all around the mouth of an earthenware vessel and thereby shields whatever is inside it from becoming tamei. Our mishnah continues the discussion of a barrel closed with a stopper, and it asks about a stopper that is not sitting altogether firmly in place.
The mishnah opens: "Megufat hechavit hamechulchelet ve'eina nishmetet" - a stopper of an earthenware barrel that is loose in its place, and yet does not fall away from the opening it is closing. Picture the scene: the plug really is sitting in the mouth of the barrel, it has not come free, but it was never wedged in snugly. Handle it and it shifts slightly. Even so, it grips well enough that were the barrel tipped over onto its side, the plug would remain where it is.
Is such a closure good enough to be called a tzamid patil? "Rabbi Yehudah omer, matzelet" - Rabbi Yehudah holds that it does shield the contents. In his view, although the stopper is shaky and not fixed in tightly, since it stays in the mouth of the barrel it counts as a cover sealed all around, and the food and drink inside are protected from becoming tamei.
"Vachachamim omrim, einah matzelet" - the Chachamim disagree. To their mind, a plug that is loose has not truly sealed the vessel. A wobbling stopper is not a tzamid patil, and therefore the contents inside the barrel are not protected and do become tamei.
When the finger hole sinks into the barrel
The mishnah now moves to a second situation: "Hayah beit etzba shelah shokei'a betochah" - the finger socket of the stopper was sunk down inside the barrel's opening. Some explanation is in order. A plug has to be removed from the barrel now and again, so it was fashioned with a small hollow in which a finger could be placed, providing the grip needed to lift the cover away. Our case is one in which that hollow is positioned low, so that even while the plug is closing the vessel, the socket itself descends below the rim and comes to lie within the barrel's airspace.
Given that arrangement, the mishnah draws two conclusions. "Hasheretz betochah, hechavit temei'ah" - should a sheretz make its way into that finger hollow, the barrel is tamei. Strictly speaking the sheretz is resting in the plug and not in the vessel, but since the plug's hollow lies within the barrel's airspace, the sheretz is treated as being inside the vessel, and whatever the barrel holds contracts tumah.
And the reverse: "Hasheretz bechavit, ochalin shebetochah temei'in" - if the sheretz is inside the barrel, then food lying in that finger hole becomes tamei. Here too the reason is the same. Because the socket is sunk deep, whatever food is resting in it is really within the barrel's airspace, and so it is reached by the tumah of the sheretz below.
The lesson of the mishnah, then, runs in both directions. A cover that sinks into the vessel does not simply sit above it as a barrier; part of it has entered the vessel's own space, and whatever rests in that part is judged as though it were inside the barrel itself, whether it is a source of tumah or food susceptible to it.