Keilim, Chapter 12, Mishnah 4. This chapter examines small metal items and parts of larger devices, asking in each case whether the piece before us counts as a keli in its own right, capable of receiving tumah, or whether it is merely an accessory to something else. Our mishnah brings four such cases: a pin used by a bloodletter, a pin set into a sundial, a pin belonging to a weaver, and a storage cabinet used by those who ground beans.
The mishnah's first case is stated in three words: "masmer hagara tamei". The gara is one who lets blood, and his masmer was a small sharp blade with which he would puncture the patient's arm in order to draw blood out of it. Such an instrument can contract tumah. Notice that it holds nothing whatsoever and encloses no space, yet it is metal, and for metal that is no obstacle: a metal utensil becomes susceptible without any receptacle at all, so long as it genuinely serves a purpose in a person's hands.
The mishnah continues: "veshel even hashaot tahor" - but the pin of a sundial is tahor. A sundial of that period was a metal rod into which pins were set; as the sun moved across the sky, the shadow falling on one pin or another indicated the hour of the day. The pin, then, is nothing more than a marker. It performs no motion and does no work of its own, and therefore, according to the first opinion, it is not treated as a keli that can receive tumah.
"Rebbi Tzadok metamei" - Rebbi Tzadok disagrees and rules that the sundial's pin can indeed become tamei. In his view, the fact that the pin does not move is not decisive. It does carry out a real function, namely marking the hours, and that function is enough to give it the status of a keli.
Next comes the pin used in the weaver's trade. Here we are speaking of a lengthy metal rod that the weaver drove into a wooden spool, around which the thread was then wound, the two together doing their job at the loom. The ruling: "masmer hagardi tamei", this pin too can contract tumah. One might have thought otherwise, for the rod is fixed into wood, and that wooden spool cannot itself become tamei since it encloses nothing. Nevertheless the rod does not lose its identity in the wood that surrounds it. It is viewed as the essential element of the implement, and so it keeps its standing as a metal keli in its own right.
The last case involves a cabinet particular to one trade: the garosot, workers who ground beans. After the grinding was done, the produce was kept in this cabinet, a bulky and weighty piece of furniture that no one would carry from place to place. On this the mishnah records a dispute, opening with "ve'aron shel garosot, Rebbi Tzadok metamei". Rebbi Tzadok maintains that the bean grinders' chest does become susceptible to tumah, for it is made of wood and it holds its contents inside it.
"Vachachamim metaharin" - the Chachamim rule that it is tahor. Their reasoning is that this cabinet was never intended to be carried about. The halachah, derived from the pesukim, is that a wooden keli which is not designed to be moved cannot become tamei, no matter how fine a receptacle it may have.
One more detail is appended. The cabinet rested on top of a wagon, but not so that it could be wheeled anywhere; the wagon simply held it steady where it stood, and the cabinet stayed put. Of this the mishnah rules: "hayeta agala shela", if the wagon beneath it was "shel matechet", made of metal, then "temei'ah", that wagon is susceptible to tumah. Because it does not serve as an auxiliary of the wooden chest above it, we assess it independently, and as an implement of metal it is able to contract tumah.
So this mishnah teaches us to look carefully at every piece of an implement and ask what it truly is: a working metal keli, a mere marker, the main part of a device, an immovable wooden fixture, or an independent object that happens to be standing underneath something else. The answer to that question determines everything.