Parah, Chapter 12, Mishnah 5. This chapter deals with the practical details of sprinkling the mei chatas, the water containing the ashes of the parah adumah, and here the Mishnah discusses a man purifying a tamei utensil while taking care not to become tamei himself, and then defines exactly how much water constitutes a sprinkling.
The case: "ochez hu hatahor bekardom hatamei bichnafo umazzeh alav". A tahor person needs to sprinkle on a tamei ax. Rather than take hold of it with his bare hand, he grasps it through the corner of his garment so that he never touches the tamei metal directly, and in that position he sprinkles the mei chatas onto it.
And the halachah? The Mishnah's verdict is "tahor": the man who held the ax is not rendered tamei. This is so, says the Mishnah, "af al pi", notwithstanding, that liquid has settled on the blade in a volume that would still be enough for a sprinkling, a "kedei hazayah". Since his skin never came into contact with the metal, the tumah of the ax has no path by which to reach him.
Having just hinged its ruling on the volume of water left behind, the Mishnah now stops to fix that volume precisely. It asks: "kamah yehei vamayim", what quantity of water must be present, "vihei vahen kedei hazayah", for it to qualify as an amount fit for sprinkling? The answer given is "kedei sheyitbol": there must be enough liquid that a person could immerse in it the "roshei gevollin", the very ends of the shoots of the eizov, the hyssop with which the sprinkling is done, "veyazzeh", and then perform the sprinkling with them.
Rebbi Yehudah proposes a different yardstick: "ro'im osam ke'ilu hein", we assess the water as if it were lying "al eizov shel nechoshes", on a stalk of hyssop fashioned out of copper. Why copper? Because metal draws in nothing at all. Genuine hyssop, being a plant, drinks in a share of the water into which it is dipped, so a portion of the liquid is swallowed by the stalk and never reaches the object being sprinkled. A copper stalk, by contrast, retains everything on its outer surface, so the entire measure stays usable for the hazayah. Rebbi Yehudah thus calculates the shiur with no margin added for what a plant would soak up. The accepted halachah, though, is not in accordance with Rebbi Yehudah; the measure is taken according to the Mishnah's opening view.
So this Mishnah teaches us both a practical technique, purifying a tamei implement while keeping oneself clear of its tumah by holding it through one's garment, and a precise standard: a quantity of mei chatas counts as a hazayah when the tips of the hyssop's shoots can be dipped into it and a sprinkling performed with them.