Keilim, Chapter 18, Mishnah 7. This mishnah takes up a bed leg that had been removed from its bed, became tamei on its own while detached, and was then fitted back onto a bed that was tahor. The question is what happens to the bed while the leg is joined to it, and what remains once the leg is pulled off again. The Mishnah answers this in three stages, according to the level of tum'ah that the leg carried.
Case One: A Leg That Carried Tum'at Midras
The Mishnah's first clause reads "kera shehayeta teme'ah midras", a bed leg bearing midras tum'ah, and then "vechiberah lemitah", that someone joined it back onto a bed. Visualize the sequence: the leg is sitting by itself, apart from the frame, a zav rests his weight upon it and it thereby acquires tum'at midras, and only later is it set back into place as part of the bed.
The ruling: "kullah teme'ah midras," the whole bed now carries tum'at midras. The reason is the general principle that whatever is joined to a tamei object takes on that tum'ah: kol hamechubar latamei, tamei. Since the leg is now part of the bed, the midras status of the leg spreads through the entire vessel.
"Perashah, hi teme'ah midras vehamitah maga midras": if the leg is detached once more, the leg itself keeps its tum'at midras, but the bed is now only a rishon, a vessel that came into contact with a midras. Once the connection is broken, the bed no longer shares in the leg's status as an av hatum'ah. Still, it did touch that av hatum'ah while they were joined, and so it is left with the tum'ah of one who touched a midras: a rishon latum'ah.
Case Two: A Leg That Carried Tum'at Met
The Mishnah's second clause: "hayeta teme'ah tum'at shiv'ah", the detached leg had picked up the seven day tum'ah, which is tum'at met, corpse tum'ah, rather than midras. Then "vechiberah lemitah", it was fastened onto a bed that until now had been tahor. The verdict is "kullah teme'ah tum'at shiv'ah": the bed in its entirety now holds that same seven day tum'ah, since it is bound together with a leg that bears corpse tum'ah.
And when they are separated? "Perashah, hi teme'ah tum'at shiv'ah vehamitah teme'ah tum'at erev." The leg holds on to its corpse tum'ah of seven days, while all that clings to the bed is tum'at erev, an impurity that expires at nightfall. Put differently, the bed is a rishon, since it had been in contact with something bearing tum'at met.
Case Three: A Leg That Was Only a Rishon
The third clause presents the identical sequence with one change: this time the detached leg had never been an av hatum'ah at all, but only a rishon. In the Mishnah's words, "hayeta teme'ah tum'at erev", the leg carried nothing more than tum'ah until evening, and then "vechiberah lemitah", he joined it to the bed. The ruling: "kullah teme'ah tum'at erev." Every part of the bed acquires precisely the level the leg holds, no more and no less: tum'at erev.
"Perashah", they are pulled apart once more, and now: "hi teme'ah tum'at erev", the leg retains the tum'ah it had until evening, "vehamitah tehorah", while nothing at all remains on the bed. Contact between the two is not enough to change this. A leg that stands merely at the level of rishon has no power to transmit tum'ah to a vessel, for kelim receive tum'ah exclusively by touching an av hatum'ah, and never from a rishon latum'ah.
The Prong of a Ma'ader
The Mishnah closes: "vechen hashen shel ma'ader", and the very same rules apply to the tooth of a pitchfork. Such an implement consists of iron teeth driven into the upper end of a wooden shaft. Should a tooth become tamei while it is lying detached, and afterward be reinserted into its handle, the tool as a whole assumes exactly the level of tum'ah borne by that tooth, be it the level of an av hatum'ah or merely of a rishon, just as we saw with the leg and the bed.
So too in the reverse direction. Pull the tooth out, and the tooth holds on to whatever tum'ah it had at the outset, while the tool is judged as an object that came into contact with it. Where the tooth held the status of av hatum'ah, the pitchfork is left as a rishon. Where the tooth held nothing beyond the level of rishon, the pitchfork emerges tahor, for the reason already established: a utensil takes tum'ah from touching an av hatum'ah alone, and a rishon latum'ah cannot pass anything on to it.