Keilim, Chapter 9, Mishnah 6. Our mishnah is built of two separate pictures that share one underlying idea: what happens when something that could be tamei is absorbed inside something else. The first half deals with oil and wine that were soaked back up by the pulp and the skins they were pressed from. The second half deals with a piece of metal that has sunk into wood or into clay and is now hidden inside it.
Oil and wine that returned into the pulp
The mishnah opens with "hagefet vehazagin shena'asu vetaharah": crushed olive residue and the skins left over from grapes, whose oil and whose wine had been produced under conditions of taharah. That pressing was carried out in purity from beginning to end, pure workers handling pure vessels, so that what came out of the press was tahor as well. Yet a press never extracts everything. The moisture that stayed behind was soaked up once again by that pulp and by those skins.
The mishnah goes on: "vehalchu aleihem teme'im", tamei people trod upon this pulp and these skins, and then "ve'achar kach yatze'u meihen mashkin", their weight forced moisture back out of them. The verdict is "tehorin": what emerged is pure, "shemitechilatan na'asu vetaharah", because its original production had taken place in a state of purity. Having been reabsorbed while tahor, it retains that status even when the treading of a tamei person presses it out.
Two clarifications are in order. To begin with, when someone tamei walks barefoot on food or on liquid, he does render it tamei, since his foot makes direct contact with it. Olive residue and grape skins, however, do not count as food, so treading upon them, or shifting them about, leaves them exactly as they were: tahor.
The second question is why the moisture squeezed out under his feet escapes tumah. The reason is that juice pressed from a fruit, olive oil or grape juice included, attains the halachic standing of a liquid capable of receiving tumah only when it is drawn out by intent. Seepage that happens on its own, wanted by nobody, has no standing as liquid at all and is not readied to receive tumah. In our case the tahor workers did extract the oil and the wine on purpose, and at that stage the produce did carry the standing of liquid. But once it soaked back into the pulp and skins it reverted to what it had been before: a component of the olive, a component of the grape, no longer liquid in its own right. Only a deliberate pressing can turn it into liquid once more. The tamei man had no such intent whatsoever; his purpose was to walk across, not to extract, and he would rather nothing came out at all. Consequently, although moisture is genuinely seeping out beneath his feet, it holds no status as liquid, and something without that status cannot contract tumah.
A metal keli swallowed by wood or by clay
The mishnah now moves to a solid item that has become lodged inside another object. It begins with the spindle and its hook. A spindle is what one uses to twist fibers into thread, and a metal hook was fixed at its end to catch hold of them. Soft wood allows such a hook to work its way further and further in, until the wood closes over it completely, and yet it has not stopped being a full metal keli in its own right.
Hence the wording: "kush shebala et hatsinorah", a wooden spindle which swallowed up the metal hook mounted at its end; "malmad shebalat hadarban", a goad which swallowed its spike. The malmad was a wooden staff bearing a metal point, the darban, with which one urged an ox forward so that it would keep walking and plow a straight furrow. Its wood too is soft, the point works its way inward until it is entirely covered over, and we nevertheless treat that point as a self standing, complete metal keli. "levenah shebal'at hataba'at": a metal ring dropped into clay, was worked into it, and the clay then hardened into a brick with the ring inside.
In each of the three, "vehen tehorin", the hook, the point and the ring had all been pure when the surrounding material closed over them. Then, teaches the mishnah, "nichnasu le'ohel hameit nitma'u": they were carried into a tent where a corpse lay, and tumah reached them, notwithstanding that their casing is itself immune to tumah. A spindle and a goad are flat wooden implements with no receptacle, and implements of that sort never become tamei; a brick is flat earthenware and likewise cannot become tamei. All the same, their purity affords the metal inside them nothing. Against corpse tumah in a tent the one effective barrier is a sealed earthenware vessel, and neither a wooden shaft nor a brick can screen off a metal keli buried within it. The metal therefore contracts tumah exactly where it lies.
"Hesitan hazav nitma'u": should a zav shift these items, tumah takes hold of them, once again despite their being encased in something not susceptible. A zav transmits tumah to objects through moving them, with no need for actual contact. Thus if a keli is bundled up or embedded within another object and a zav sets it in motion, the hook, the point or the ring inside comes out tamei.
"Naflu la'avir hatanur tahor, time'uhu": once such an item has contracted tumah, whether by way of the corpse or through the zav's movement, should it then drop into the airspace of a tahor oven, the oven becomes tamei, even with the metal still encased in wood or in clay, since it has entered the space enclosed by the oven's walls. A keli that took tumah from a corpse holds the rank of av hatum'ah, and so does one that a zav moved; and an av hatum'ah that comes into an oven's airspace conveys tumah to that oven.
The mishnah ends: "naga bahen kikar shel terumah, tahor". Terumah bread that touched them, meaning that it touched the wood or the brick housing that tamei metal, remains pure. Contact was made only with the surrounding wood or clay, never with the tamei keli itself. And although terumah is judged by more demanding standards than ordinary food, here too the loaf stays tahor.
Both halves of the mishnah teach us to look carefully at status rather than at appearance. Liquid that has sunk back into the pulp is once again part of the fruit and only regains the standing of liquid when someone deliberately extracts it. Metal that has sunk into wood or clay, by contrast, never loses its standing as a complete keli, and the material around it offers it no shelter from tumah, nor does it pass that tumah on to whatever touches it from outside.