Keilim, Chapter 13, Mishnah 8. The subject here is a comb, the kind used for combing out the fibers of flax and of wool. Its handle is made of wood, yet the comb is classified as a metal keli, because some of its teeth are made of metal. That classification matters a great deal, since a metal keli is susceptible to tumah even when it is completely flat and has no receptacle at all. The Mishnah now examines what becomes of such a comb once some of its teeth are gone, and what becomes of the teeth themselves once they are separated from it.
The flax comb
The first case is a flax comb: "Masrek shel pishtan." Suppose its metal teeth were pulled away, "shenitlu shinav," until nothing was left but a pair of them, "venishtayer bo shtayim." The ruling is "tamei": the comb is still susceptible to tumah. A pair of teeth suffices for the combing to be done, and a utensil that still functions has not forfeited its identity as a keli.
"Ve'achat, tahor." But if only a single metal tooth is left, the comb is no longer usable for combing at all, and it therefore no longer counts as a keli and cannot receive tumah.
Attention now shifts from the comb to the teeth that fell away from it. "Vechulan," all of them, "achat achat bifnei atzman," viewed one by one on their own, are "temei'ot." Even when not a single tooth is left in the handle, every one of them can receive tumah, because a lone tooth of a flax comb serves as a stylus with which letters are traced into the wax of a writing board. Having a function of its own, the loose tooth stands as an independent keli.
The wool comb
Now the wool comb: "Veshel tzemer shenitlu shinav," a comb for wool whose teeth were extracted, "achat mibeintayim," one taken and the next left, alternating across the row. Such a comb is "tahor." Spaced out that way, the teeth can no longer accomplish the combing, and a utensil that cannot perform its function is not a keli.
The Mishnah continues: "Nishtayer bo shalosh," three teeth survived, "bimkom echad," all in one place, adjacent to one another. Then the comb is "tamei." A cluster of three neighboring teeth is sufficient for the combing to proceed, and since the comb still works, its status as a keli is intact.
"Hayetah hachitzonah achat mehen, tahor." Should one member of that trio be the tooth at the very edge, the ruling is that it is tahor. That outermost tooth is made thicker than its neighbors and takes no part in the actual combing, so it cannot be counted among the three that are needed for the utensil to remain in working order.
Reworking the detached teeth
"Nitlu mimenu shtayim va'asa'an lemalketet, temei'ot." If two teeth were taken out of a wool comb and their owner fashioned them into a pair of tweezers for plucking hair, they are susceptible to tumah, because he has turned them into a working tool. Note the difference from the flax comb: the teeth of a wool comb are too thin to serve as a stylus for writing, yet they have other uses, and once they are put to such use they are keilim.
Finally, a single tooth: "Achat vehitkinah," one was detached and its owner adapted it, "lener o lemitu'ach," to serve in tending a lamp or to draw cloth taut. The verdict is "temei'ah." Having been fitted for a job, that one tooth is now a functioning implement and can receive tumah.
The thread running through the whole Mishnah is a single principle: susceptibility to tumah follows usefulness. So long as enough teeth remain for the comb to comb, it is a keli; when the remaining teeth cannot do the work, it is tahor. And a broken piece that has a use of its own, whether naturally, like the flax comb's tooth that writes on wax, or by the owner's design, like the tweezers, the lamp cleaner and the cloth stretcher, is not mere scrap metal but a keli in its own right.