Taharos, Chapter 2, Mishnah 1. The second chapter opens with a mishnah that is, in truth, a review and reapplication of a whole series of principles built up in the first chapter: the ladder of degrees of tumah, the shiur of a keveitzah needed for food to convey tumah, and the strange behavior of liquids, which leap to the status of a rishon. The setting is entirely domestic: a woman pickling vegetables in a pot of brine.
The mishnah's first words are "Ha'ishah shehaysah koveshes yarak bikdeirah": someone at home is preserving greens in a vessel of salt water. Two features of the scene matter halachically: there is liquid sitting inside the vessel, and the produce is steeping in that liquid. That the case is framed around a woman carries no legal weight at all; it is simply the natural picture of a household kitchen, and niddah plays no role whatsoever here. What does matter is the halacha of hands. Hands whose recent history is unknown to us, stam yadayim, are assigned the rank of shniyos. This is not because hands are intrinsically tamei; it is because we cannot vouch that they stayed clean, and so we treat them as bearing sheini-level tumah.
Touching a dry spot
The case continues: a leaf or stalk protrudes above the surface of the brine, and she touches the leaves, "venag'ah be'aleha", at a point outside the vessel that happens to be dry, "bimkom hanaguv". Keep in mind that produce can receive tumah only after hechsher, once it has been wetted by a liquid; this vegetable certainly was wetted, so it is susceptible, yet the precise place her hand met is dry at this moment. The ruling: even where the piece contains a keveitzah, "hu tamei vehakol tahor", that piece alone is tamei and all the rest stays clean.
Why? These are vegetables of terumah. Her hands are shniyos, so what she touches becomes a shlishi. A shlishi of terumah is pasul in itself, yet it has no power to convey tumah to any other terumah. Now, one might have expected the brine below to jump to the status of a rishon, since liquids always leap to first degree. But here the principle we have already established applies: a food that cannot render another solid tamei cannot render a liquid tamei either. The shlishi is spent. So even if the vegetable has a keveitzah, the full shiur, it alone is tamei and everything else in the pot remains tahor.
Touching a wet spot
Next case: "nag'ah bimkom hamashkeh", her hand lands on a damp part of the leaf. That is an entirely different matter, since the contact is now with liquid rather than with a dry solid, and liquid vaults directly to first degree. That rishon liquid then makes the vegetable a sheini instead of a shlishi. A sheini is capable of passing tumah onward, but only when it holds the minimum measure. So the mishnah rules that if the piece amounts to a keveitzah, "hakol tamei", the whole lot becomes tamei; and if it falls short of that measure, it has no power to transmit, so it alone is tamei while everything else keeps its taharah.
Returning it to the pot
"Chazar lakdeirah, hakol tamei." Suppose that protruding piece is now pressed back down beneath the surface. A fresh principle enters: liquids have no minimum measure whatsoever. The film of moisture clinging to the leaf is a negligible quantity, yet the instant it merges with the liquid already standing in the vessel, that entire body of brine takes on its status. The brine is a rishon; it makes the vegetables tamei, and it even makes the vessel tamei, a halacha taught in Zavim, that liquids convey tumah to keilim.
If she herself is a rishon
Now change her own status: "haysah maga tmei meis", she has come into contact with a corpse. Her hands are no longer the issue, since her entire person now ranks as a rishon. Under those circumstances the wetness of the contact point makes no difference, "bein bimkom hamashkeh", at a damp place, "bein bimkom hanaguv", at a dry one: whatever she touches becomes a sheini immediately. The one remaining question is quantity. If the piece amounts to a keveitzah, it is substantial enough to pass tumah onward, and everything (the contents of the vessel and the vessel too) becomes tamei. If it holds less than that, it is tamei on its own and transmits nothing, leaving all the rest tahor.
The tvulas yom and the doubtful liquid
The mishnah's final clause introduces several new factors at once. The woman here "haysah tvulas yom": she has already immersed in the mikveh and is only waiting for sunset to be completely tehorah, and throughout that interval she ranks as a sheini. She thus carries two separate tumos simultaneously, since "veyadayim meso'avos", her hands are soiled as well, and each of the two has its own set of rules. And she is "mena'eres es hakdeirah", shaking up what the vessel holds.
"Vera'asah mashkeh al yadeiha": she then notices liquid on her hands and does not know where it came from. "Safek min hakdeirah nitzu": perhaps it splashed up from the pot, in which case she never touched liquid that was in contact with the vegetables, and everything would be tahor. "Safek im hakelach nag'ah beyadeiha": or perhaps the stalk itself came into contact with her hands while it mattered, and she conveyed tumah to everything.
The ruling: "Hayarak pasul vehakdeirah tehorah." How is the doubt resolved? Begin with the doubt regarding her hands. A safek about hands is tahor, because hands are not inherently tamei; their status is only a matter of our not knowing. So when a further doubt is introduced, whether the hands even made contact, we are left with a doubt piled on a doubt and we can dismiss it.
The tvulas yom, however, presents a genuine difficulty. That safek bears on a Torah-level matter, so we must rule stringently, and the vegetables are disqualified. Why, then, does the pot stay tehorah? Because a woman in the state of tvulas yom lacks the power to raise liquid to first degree. Every other source of tumah that meets a liquid pushes it up to rishon; the tvul yom is singled out as the exception, as stated in Mishnah Parah. Since she could not have turned the brine into a rishon, nothing was present that could have made the vessel itself tamei or pasul. The result: the vegetables come out disqualified, and the pot remains tehorah.