Parah, Chapter 6, Mishnah 4. This chapter is occupied with the many ways in which the water for the mei chatas can be ruined, and here the Mishnah turns to the moment of filling: how exactly does the water get from the spring into the jar? The Mishnah teaches that the answer matters a great deal.
The first half of the Mishnah lists three failures. "Nosan yado," he stretched out his hand, "o raglo," or else his foot, "o alei yerakos," or he arranged leaves of vegetables, "kedei sheya'avru hamayim," so that the flowing water would travel across them and drop "lechavis," into the jar below. Of all three arrangements the Mishnah rules "pesulim": water drawn in such a manner is unfit for the mei chatas.
"Alei konim v'alei egoz, ksherim." If instead he used reed leaves or the leaves of a nut tree as the channel, the water remains kosher.
The Mishnah does not leave the distinction for us to work out on our own; it spells out the governing principle: "Zeh haklal: davar shehu mekabel tumah, posul; v'davar she'eino mekabel tumah, kasher." Everything hinges on the nature of the channel. Should the liquid have traveled across an object that is able to become tamei, it is disqualified; should it have run over something that never takes on tumah at all, it remains fit. A person's limb, hand or foot alike, is subject to tumah, and leaves of vegetables are food, which is likewise susceptible. Reeds and the foliage of a nut tree, by contrast, stand outside the categories of tumah entirely, so water that slid along them on its way into the jar suffers no harm.
Where the Rule Comes From
Where does this yardstick come from? From the Torah's language concerning immersion: "mikveh mayim yihyeh tahor," an accumulated body of water retains its taharah. The Chachamim understood the pasuk to be addressing more than the pool as it already stands; it speaks as well to how the water arrived there in the first place. The very formation of the mikveh, the route by which it is filled, must be accomplished through taharah, and not through anything liable to become tamei.
Although the verse is written with regard to a mikveh, the principle is extended to purifying water generally. Any water that brings about taharah, whether it is gathered in a mikveh or drawn for the sanctification of the mei chatas, must reach its place by way of something that is itself beyond the reach of tumah. Hence the two lists in our Mishnah: a hand, a foot, or vegetable leaves disqualify the water, while reeds and nut leaves may serve as its channel.