Keilim, Chapter 2, Mishnah 5. This chapter is occupied with vessels of earthenware and the question of which of them can receive tumah at all. Our mishnah turns to a family of objects that sit right on the border: covers. A cover is not a container, it is what closes a container. Yet many earthenware covers were shaped in a curve or hollow on the inside, so that the moment you turn them over they look exactly like a little bowl. Are they vessels or not?
The Mishnah begins with the covers of jugs: "kisuy kadei yayin vechadei shemen," the covers of wine jugs and of oil jugs, "vechisuy chaviyot neyarot," and the covers of the barrels called neyarot, which were tall and hollow inside and could likewise hold something once inverted. Regarding all of these the ruling is "tehorin," they are tahor and cannot receive tumah.
Why not? Because their entire purpose is to be set with the hollow side facing downward, over the mouth of the jug. Even though the shape would allow it, this object is not meant to serve as a container. And for earthenware the Torah is precise: a vessel of earthenware becomes tamei only when the tumah enters "el tocho," into its inside. That wording tells us that the inside must be an inside that has a use. Here the inside of the cover has no function of its own; the object is nothing more than a covering.
The Mishnah then adds a qualification: "ve'im hitkino letashmish, teme'in." Should the owner designate one of these covers to function as a receptacle, it enters the category of vessels that contract tumah. Consider what that designation requires. No hole is bored, nothing is trimmed away, no new shape is fashioned, and the object still closes a jug whenever he wishes. What has shifted is only his mind, a resolution about the role the piece will play from here on. That resolution alone endows the hollow with a purpose, and the cover becomes a keli.
From jug covers the Mishnah passes to the lid of a stew pot: "kisuy halefas," whose form curves inward. Two features keep it outside the laws of tumah: "bizman shehu nakuv," when it is perforated, or "veyesh lo chidud," when a sharp point projects from it. In either situation the ruling is "tahor." Each of those features makes the lid useless as a receptacle even after being flipped over. The perforation was made to let the steam of the cooking escape, and a lid with an opening in it retains nothing, since whatever is poured in runs straight out. A raised point at the center leaves no level base for the inverted lid to rest upon, so it topples instead of serving as a dish. A lid of that sort remains pure permanently and has no possibility of ever contracting tumah.
The reverse case follows: "ve'im eino nakuv," and if there is no perforation, "ve'ein lo chidud, tamei," and no point rises from it, so that inverting it yields a steady, level surface, then it is susceptible to tumah. The reason offered is "mipenei shehi mesanenet," because the cook drains, "letocho et hayarak," the vegetables into it. In the course of cooking she lifts the vegetables out and rests them on the upturned lid so the liquid runs off them, and this is standard kitchen practice. Hence, although the piece was made in order to close a pot, that ordinary act of draining is a genuine receptacle use, and it renders the lid a keli capable of becoming tamei.
Rabbi Eliezer bar Tzadok points to a different routine task, "mipenei shehi hofechet alav et harunki": the lid ranks as a receptacle because the woman of the house tips the runki out onto it in order to crush it. Runki means vegetables boiled until they cling together as a single soft lump. She would invert the lid, empty that mass upon it, and mash it there.
No argument divides the two opinions. The first Tanna and Rabbi Eliezer bar Tzadok hold alike that a smooth, unpierced pot lid is susceptible to tumah; they differ only in identifying which domestic task lends the hollow side a purpose of its own. Once that hollow genuinely serves a purpose, the object has outgrown the status of a mere lid. It is a vessel, and every vessel stands under the halachot of tumah and tahorah.