Keilim, Chapter 14, Mishnah 8. This chapter deals with metal implements and the question of when a piece of metal still counts as a keli, a utensil that can receive tumah, and when it has become so damaged that it is simply a scrap of metal. Our mishnah applies that principle to several everyday objects: two kinds of keys, a strainer used for mustard, and the funnel that feeds grain into a mill.
The knee-shaped key
The mishnah's first case concerns a key modeled on a human leg. A leg turns at the knee, and turns a second time where it meets the foot, and this key was constructed with the same double angle. It consisted of three parts: the portion held in the hand, an angle known as the "arkuba" (the knee), and beyond that a final stretch that turned yet again, corresponding to the foot. That last portion, past the knee, was what entered the lock and did the actual work.
The halachah is stated thus: "mafte'ach shel arkuba", a knee-shaped key, "shenishbar mitoch arkubato", which fractured right at the knee itself, the spot furthest removed from the grip, is "tahor". Why should that be? Because with that end missing, the key can no longer release a lock for a person standing outside the door, and unlocking from outside is exactly what keys are made for. It may still manage to shift a bolt for someone standing within, yet that was never its principal task. Stripped of the use for which it was fashioned, the surviving fragment is no longer counted as a keli, and what is not a keli is not susceptible to tumah.
Rabbi Yehuda takes the opposite view. He rules "Rabbi Yehuda metamei", the broken key does become tamei, "mipenei shehu pote'ach bo", because a person can still work a lock with it, "mibifnim", from within the house. In Rabbi Yehuda's reckoning, that leftover usefulness carries the day: the object continues to serve its owner, and so it keeps the standing of a utensil.
The gamma-shaped key
The mishnah passes now to a key possessing one angle rather than two. Its name is borrowed from gamma, a letter of the Greek alphabet whose form resembles the letter L turned upside down. Picture it: a grip, one single turn, and then the stretch that enters the lock.
Of that key, "veshel gam", the mishnah teaches that if it "shenishbar mitoch gumo", broke at the very place where it turns, then it is "tahor". On this ruling Rabbi Yehuda raises no objection, for a fracture at the angle leaves nothing behind except the bare grip, and a grip on its own achieves nothing whatever. Neither a person outside nor a person inside can operate a lock with it. Since not a single use survives, not a single trace of a keli survives either.
A key with teeth and with holes
Some keys were made with two different mechanisms on the working end: projecting teeth, which enter the lock and push its parts, and holes, into which pegs of the lock enter. Both were means of releasing a lock, each suited to a different kind of lock.
"Hayu lo chafim unkavim, tamei." If the surviving section of the gamma key still has its teeth and its holes, it can receive tumah, even though the handle is gone, and even if part of the teeth or part of the holes are missing. What matters is that the business end can still do its job.
The mishnah proceeds to weigh each mechanism on its own. Should the teeth be broken away, "nitlu chafim", the ruling is "tamei mipenei nekavim", it remains susceptible on the strength of the holes, since locks built around pegs can still be opened by it. And should the holes close over, "nistatmu nekavim", the ruling is "tamei mipenei chafim", it remains susceptible on the strength of the teeth, for locks built around teeth are still within its power. In both directions a genuine use survives, and a surviving use keeps the object within the category of a keli that takes on tumah.
When both mechanisms are ruined at once, however, the picture changes. "Nitlu chafim", the teeth broken away, together with "venistatmu nekavim", the holes plugged shut, or else the case where teeth and holes were pressed and bent until, in the mishnah's words, "sheparsu zeh letoch zeh", they spread over into each other and run together: in all of these the key is "tahor". Inserting it into a lock will accomplish nothing at all, and where no function whatsoever remains possible, no keli remains.
The mustard strainer
The mishnah now moves to a different kind of object whose holes have merged, this time a metal strainer used for mustard. Mustard seed comes mixed with chaff, the unwanted husk, and the strainer's holes were made small enough that the seeds would drop through while the chaff was held back above.
The mishnah's wording is "mesannenet shel chardal", a mustard strainer, "shenifresu bah sheloshah nekavim lematah zeh letoch zeh, tehorah". Should three of the perforations along its floor tear open into one another and become one wide mouth, the whole vessel is pure. A gap that broad allows the husks to tumble down together with the seed, so nothing is being separated any longer. The strainer has forfeited the very task for which it was made, it has ceased to be a utensil, and tumah has nothing left to take hold of.
The funnel of the mill
The concluding case concerns the hopper that sits above a mill. Grain is poured into the broad opening on top and seeps out through the constricted spout at the bottom, dropping down to be ground. Because it emerges bit by bit, the milling is done properly, instead of the whole quantity rushing in at once.
"Veha'afarkes shel matachot, temeiah." A hopper of this kind fashioned from metal is susceptible to tumah. One might have argued the reverse: since its sole task is delivering grain to the millstones, perhaps it should be reckoned as nothing more than a part of the mill, with no independent standing of its own. The mishnah rejects that reasoning. This object bears a name of its own, people identify it as an afarkes without any need to mention the mill, and therefore it holds the standing of a keli that becomes tamei.
Running through all five cases is a single measure. A metal object is a utensil for as long as it can still do work for a person. A key that lost the part that opens a door from the outside, a key whose teeth and holes have both been ruined, a strainer whose holes have merged into one wide gap: these have been emptied of purpose and are tahor. A key that retains even one working mechanism, and a funnel that still guides grain into the mill under its own name, remain utensils, and remain subject to tumah.