Keilim, Chapter 17, Mishnah 2. This chapter is occupied with the measurements that decide when a vessel is still a vessel. A container that develops a hole loses its status of tumah once it can no longer do the job for which it was made, and the Mishnah keeps asking: how large a hole is too large? Here the Mishnah turns to vessels that were made to hold rather large items, items bigger than a pomegranate, and it asks about a case where the vessel broke somewhat and can no longer hold the specific item it always held, yet it can still hold something quite close to it in size. Does such a vessel remain a utensil?
The skin bottle and the two kinds of thread
The Mishnah's first example needs a picture of how such a bottle was made. When an animal's carcass was skinned in one piece, the hide could be turned into a bottle, a chemet, by stitching shut all the openings that the skin naturally had. The result was a large soft container, and one common use for it was storing balls of spun thread.
Weaving uses two kinds of thread. The sheti, the warp, runs vertically in the loom, and the erev, the weft, runs horizontally across it. The warp threads were spun thicker and stronger than the weft threads, and the balls wound from them were correspondingly larger than the balls of weft thread. Our skin bottle was normally used to hold the balls of warp thread. Once its mouth or its seams opened up so widely that those balls slipped straight through, one might assume the bottle has stopped being a container at all and can no longer contract tumah. That assumption is what the Mishnah comes to correct.
"Hachemet shiurah befika'ot shel sheti"
The Mishnah opens: "Hachemet shiurah befika'ot shel sheti," the measurement of a skin bottle is reckoned by balls of warp thread. That is the standard by which we judge whether the hole in it has become too big for its work.
"Im einah mekabelet shel sheti, af al pi shemekabelet shel erev, temei'ah." If the hole has grown so wide that the bottle can no longer retain balls of warp thread, but it can still retain the balls of weft thread, it remains capable of receiving tumah. Now, one might have expected the reverse. The warp balls are the thicker ones, so a hole that lets them out would surely let the thinner weft balls out as well. The Mishnah is therefore describing a hole that opened in such a way that the large warp balls escape while the weft balls are still held, and it rules that even though the bottle no longer performs the very task for which it was used, the remaining task is so nearly identical to it that we still regard the bottle as doing its original work. Both jobs are the storing of balls of thread. The only difference is that one thread runs vertically in the loom and the other horizontally. The hole has grown, yes, but what the bottle still does is so close to what it always did that its status as a utensil is intact, and it can become tamei.
The plate holder
"Beit ke'arot she'eino mekabel ke'arot, af al pi shemekabel et hatamchuyim, tamei." A rack or case made for holding dishes developed a hole, so that it can no longer hold the plates it was designed for, though it can still hold the larger serving platters. It remains capable of receiving tumah. Once again the vessel does not fully perform its original function, since the plates no longer stay in it, yet holding platters is so close to holding plates that the Mishnah treats this as the same utensil doing the same kind of work, and it is tamei.
The chamber pot
"Beit hara'i she'eino mekabel mashkin, af al pi shemekabel et hara'i, tamei." A chamber pot, the vessel a person relieved himself in, cracked in a way that liquid waste now runs out of it, while solid waste is still held. It too remains capable of receiving tumah. It cannot serve every part of the purpose for which it was made, since urine no longer stays inside, but since it continues to receive what is solid, that is counted as its original use, and it is still a vessel that becomes tamei.
Rabban Gamliel disagrees
"Rabban Gamliel metaher, mipnei she'ein mekaymin oto." Rabban Gamliel declares all three of these damaged containers tahor, and his reason is practical rather than technical: people do not hold on to a vessel in such a condition. His point is not that partial function is meaningless in principle; it is that an owner wants a container that works completely. A skin bottle that spills its warp balls, a dish case that will not hold dishes, a chamber pot that leaks, these get discarded and replaced rather than kept in service. Since people do not maintain them, they have ceased to be vessels, and from that point onward, in all three cases, they cannot contract tumah.
The Mishnah has thus given us a single principle in three garments, along with a dissenting voice. According to the first opinion, when the use that a broken vessel can still perform is nearly the same as the use it was made for, the vessel has not lost its identity and remains susceptible to tumah. According to Rabban Gamliel, what settles the question is not how close the remaining use is, but whether people actually keep such an item in use at all.