Parah, Chapter 9, Mishnah 5. This chapter deals with mei chatas, the purification waters of the parah adumah, and with the various ways in which they become unfit. Our Mishnah asks a practical question: once the water has already been disqualified, what do you do with it? And what happens if an animal drinks it?
The Mishnah's opening ruling: "Mei chatas shenifselu, lo yegablem betit, shelo ya'asem takalah l'acheirim." Chatas water that has gone unfit may not be worked into soil to form mud, lest a person set a hazard in the path of other people. Against this, "Rabi Yehuda omeir, batlu": Rabbi Yehuda maintains that such water has already forfeited its status.
What is objectionable about folding the water into mud? The mud will end up in the hands of people who know nothing of its history. Unfit chatas water has not lost its power to transmit tumah to anyone who touches it, and here that impurity lies concealed inside what looks like perfectly ordinary mud. A person handling it becomes tamei without the slightest suspicion of why. It is exactly this hidden trap, the takalah, that the Mishnah comes to prevent.
Rabbi Yehuda disagrees. In his view the water is nullified once it is absorbed into the mud, and therefore there is no concern at all; the mud may be used freely.
A cow that drank the water
The Mishnah turns to a second scenario: "Parah sheshasas mei chatas", a cow that has drunk of the purification water, "besarah tamei me'eis l'eis", its meat bears tumah from that moment until the same hour on the following day, a full day and night. And "Rabi Yehuda omeir, batlu bimei'eha": in Rabbi Yehuda's view the water loses itself once it is inside the animal's belly.
According to the first opinion, the water retains its power inside the animal for a twenty four hour period. If the cow is slaughtered within that time, its meat is tamei, and one must wait out the full day before the flesh is considered clean.
Rabbi Yehuda sees no reason to delay at all. Once the liquid reaches the cow's stomach it turns rancid and undrinkable, and a liquid in that state has no capacity to transmit tumah. Slaughter the animal that very hour, and the meat is tahor.
Both halves of the Mishnah turn on the same question in two different settings: does mei chatas that has been swallowed up in something else, whether in mud or in the body of an animal, still carry its potency, or has it lost itself in its new surroundings? The Tanna Kamma treats it as still dangerous and demands caution on behalf of unsuspecting people, while Rabbi Yehuda rules that in both cases it is simply gone.