Keilim, Chapter 4, Mishnah 1. Back in the second perek we were taught a principle about an earthenware utensil from which a piece has broken off: as long as the remaining fragment can hold a certain measure of liquid while standing on its own, without needing anything else to support it, it still carries the status of a kli and can therefore receive tumah. Our mishnah now takes that principle and sharpens it. What exactly do we mean when we say that the fragment must be able to stand on its own?
The mishnah's first word is "Hacheres", the potsherd, and it speaks of a shard "she'eno yachol la'amod", one that will not stay upright and hold its liquid, "bifnei ozno", and the culprit is the handle attached to it. Imagine a clay jug fashioned with a handle on each side. The jug shatters, and one of the pieces that separates from it carries a handle along with it. Look closely at that piece: strip the handle away in your mind, and the fragment would sit firmly and contain its liquid without any trouble at all. With the handle in place, however, one side is weighted down, the piece keels over, and it will not retain the measure of liquid in question unless someone props it up against something so that the load is evened out. Note how limited the defect is: the walls and floor of the shard are perfectly serviceable for holding liquid. The whole difficulty is one leftover projection that throws it off balance.
A companion case follows. The shard is one "shehayah bo chiduda", in which the break left behind a sharp spur jutting out from below, and that spur is described as "chidud machri'o", the point that tips it over. Once again the shard itself is entirely capable of containing liquid; what deprives it of a steady footing is nothing more than the ragged spike the fracture produced.
In both of these situations the ruling is tahor. The fragment is pure and is no longer treated as a kli, because in practice it cannot stand and hold the necessary measure of liquid. We do not say that since the vessel's own body is sound and only an incidental feature is unbalancing it, we should overlook that feature. On the contrary: we disregard the distinction entirely. The fragment cannot stand, and so from here onward it is not a kli and remains tahor.
The mishnah continues: "nitla ha'ozen, venishbara chidud", the handle that was tipping it over was detached, or the sharp point that prevented it from balancing was knocked off. The ruling: tahor. The fragment stays pure, even though it is now perfectly capable of holding the required liquid while standing by itself.
Now comes the next stage. "Nitla ha'ozen, venishbara chidud": someone removes the handle that had been unbalancing the piece, or the sharp spur that had kept it from settling gets broken off. The ruling is tahor. The shard remains pure, and this is so despite the fact that it can now stand unaided and contain the requisite liquid without difficulty.
"Rebbi Yehuda me'tameh": Rebbi Yehuda disagrees and holds that once the obstacle is gone the fragment can receive tumah. He refuses to draw the comparison to a gistera, and insists that a gistera is a different matter altogether. In the cases of our mishnah, he argues, the broken piece was inherently suited to hold liquid; there was no hole in it as there is in a gistera. Something external to that function, a handle or a protruding point, was simply interfering with its ability to operate on its own. Once those interferences are removed, the piece is restored to what it always was, and it is a kli, notwithstanding the fact that it did not have that status a moment earlier. A gistera, by contrast, was pierced, and the hole stripped away its very ability to hold liquid, so that it was not a kli at all. To Rebbi Yehuda the two situations are simply not analogous.
The mishnah now presents a dispute in which the positions are reversed, with Rebbi Yehuda ruling leniently and the Chachamim stringently. "Chavit shenifchata vehi mekabbelet al dofnoteha": the base of a barrel gave way, and yet, once the barrel is tipped onto its side, its walls still contain liquid. Alternatively, a barrel "shenechleka kemin shetei arevot", one that cracked along its full height and came apart into two pieces shaped like troughs, each of which will hold liquid when it rests on its side.
"Rebbi Yehuda me'taher": in Rebbi Yehuda's view these leftover pieces are pure and do not count as kelim, since they hold nothing when set the way a barrel is ordinarily set, standing upright. "Vachachamim me'tam'in": the Chachamim hold that they do receive tumah and are kelim, because barrels that are entirely intact are also sometimes rested on their sides, and that is a perfectly acceptable position for a barrel. Since the pieces do their work in a way that is normal for a barrel, their status as a kli endures.