Parah, Chapter 11, Mishnah 1. This chapter turns to the care that mei chatas requires once it is already in hand. The water mixed with the ashes of the parah adumah is the water used for sprinkling, the water that takes a person out of tumah and brings him to taharah, and it can only do its work if it has itself been guarded from tumah. Our Mishnah presents a case of a flask left standing on its own, and asks what we may assume happened to it while nobody was watching.
Found covered. "Tzlochis shehinicha megulah, uva umatzah mechusah, psulah": someone sets down a flask of chatas water with no cover on it, and when he returns the flask is covered. The water may no longer be used. Follow the logic: lids do not settle onto flasks by themselves. Human hands must have been at work here, and the majority of people take no precautions to keep themselves fit to deal with chatas. Once we know that a person came and handled the flask, we must reckon that he lacked the taharah which chatas demands, and his contact ruined the water.
The flask left covered. "Hinicha mechusah, uva umatzah megulah": here the situation is reversed. He left the flask covered and came back to find it open. Uncovering is not, by itself, evidence that a human being was there. A person could indeed have removed the cover, but so could an animal. So the Mishnah does not disqualify the water automatically; it asks instead what an animal could have done to it.
Two possibilities occupy the Mishnah. The first is a creature that drinks: "im yecholah hachuldah", if a weasel was able to get at the flask, "lishtos heimenah", to drink from it, the water is disqualified, and Rabban Gamliel adds the snake, "o nachash", to the list. The second involves no living thing at all: if the flask stood open through the night so that dew may have come down into it, "sheyeired bo tal balaylah", the water is unfit as well. This is the accepted halachah.
Chatas water and a sealed lid. "Hachatas einah nitzoles betzamid patil." Ordinarily, a vessel standing in the same tent as a meis is protected if it is sealed with a tightly fitting lid, a tzamid patil. Chatas is different: it is not saved by such a seal. Water that already has the ashes of the parah mixed into it, or the ashes themselves sitting in a container, will become impure in a room with a corpse even though the vessel is sealed shut.
What is the source of such severity? Scripture directs that the burnt remains of the parah be deposited outside the camp in a place that is tahor, and that demand for a tahor location is what teaches us that chatas gains no protection from a tightly sealed lid.
Before the ashes are added. "Umayim she'einan mekudashin", water that has not yet been sanctified, "nitzolin betzamid patil": such water follows the standard halachah. Sealed in a vessel whose lid fits it exactly, it remains tahor even under the roof of a tent that holds a corpse. Only from the moment the ashes turn it into chatas does the stricter rule take hold.
The Mishnah has thus taught us two kinds of vigilance: reading the evidence of what may have happened to an unguarded flask, and recognizing that chatas water enjoys less protection from tumah than almost anything else we handle.