Parah, Chapter 8, Mishnah 5. Having dealt at length with the mei chatas and the waters that may or may not be used with the ashes of the parah adumah, the chapter now turns to broader principles of tumah and taharah, and in particular to the unusual power that the Sages gave to liquids that have become tamei.
The Mishnah opens with a broad ruling, best taken clause by clause. "Kol vlad hatumos": any tumah that is merely an offshoot, a level derived from tumah rather than tumah at its origin; "eino metamei keilim": such tumah lacks the power to render vessels tamei; "ela mashkeh": liquid alone forms the exception. The Mishnah then adds: "Nitma mashkeh", should that liquid itself contract tumah, "timan", it proceeds to pass tumah along to vessels.
Two expressions here call for definition. A vlad hatumah, an offspring of tumah, describes something holding tumah at the first or second remove. Set against it stands an av hatumah, tumah in its original, undiluted strength. Scripture makes vessels susceptible to the latter alone; where the tumah is merely a derived level, it finds nothing in the vessel to fasten onto, and the vessel is left tahor.
To this Torah rule the Sages attached a single exception, and it is precisely the subject of our Mishnah: liquid. Liquid touched by a derivative tumah does contract tumah, and once tamei it passes tumah on to vessels. Scripture never legislated this; it is a rabbinic stringency, and the Sages patterned it on the discharge of a zav, a liquid that the Torah itself ranks at the highest level of tumah and that does convey tumah to vessels. Wanting the fences of tumah to hold firm, they treated any tamei liquid with a corresponding severity.
What emerges is a peculiar chain, and the Mishnah dramatizes it by putting words in the vessel's mouth. "Harei zeh omer", the vessel declares to the liquid: "metamecha lo tim'uni", whatever it was that conveyed tumah to you had no power over me at all; "ve'atah timasentani", yet you have gone ahead and made me tamei. Facing the vessel directly, the derivative tumah accomplished nothing. The liquid, having drawn its tumah from that very same object, manages what its own source could never do.
The lesson of the Mishnah is therefore twofold. First, vessels are guarded from derivative tumah by Torah law, and only an av hatumah reaches them. Second, liquids stand outside that protection: once a liquid has become tamei, it carries its tumah further than the source from which it received it.