Mikvaos, Chapter 1, Mishnah 2. This Mishnah continues the discussion of the lowest of the six levels of mikvaos, and to appreciate it we need a few words of background.
Background
In Parshas Shemini, having enumerated the many things that transmit tumah, the Torah adds an exception: a spring, and a cavity in which water has gathered, stay in a state of taharah. Our masechta unfolds that verse along its two lines. First, water of this sort is itself immune to tumah. Second, it functions as an agent of purification for a person, a k'li or any object that has contracted tumah. In either case the water must be lodged in the ground and not sitting inside a vessel, and it must have accumulated of its own accord, with no human hand drawing it there.
By Torah law, a gathering of water in the ground carries the properties of a mikveh even with only a revi'is: it is not susceptible to tumah, and vessels immersed in it become tahor. The Chachamim annulled that small shiur for immersion and required forty se'ah even for keilim, yet they did not abolish the concept of a revi'is entirely. Any collection of rainwater in the ground amounting to a revi'is still retains the mikveh's basic property of not receiving tumah.
This is exactly the status of mei geva'im, the first of the sheish ma'alos b'mikvaos, the six ascending grades our chapter lists. Mei geva'im means rainwater that gathered by itself in a small pocket or depression in the earth. Resting in the ground as it does, it is beyond the reach of tumah; water becomes nitlash, detached, and thus able to contract tumah, only once a person purposely lifts it out of the earth. On the other hand, a depression of that size clearly holds nothing near forty se'ah, so it has no power to purify tamei water that arrives in it. The result is that whenever tamei water enters, we are faced with a mixture: the mei geva'im themselves are tahor, yet hidden somewhere among them is a drop of tamei water, poised to transmit its tumah the moment it is drawn up.
A new way for tumah to enter the water
Our Mishnah presents the same halachic pattern as the preceding one, but with a different source for the tumah in the water. There the tumah came from a tamei person who drank from the hollow. Here it comes from a vessel.
"Milah bichli tamei." A person drew from the depression using a k'li that was tamei. Whatever water clung to that vessel's exterior was rendered tamei by it, and a bit of that water trickled back down into the pocket below. Once more we are confronted with a bead of tamei water lodged among the mei geva'im, water which cannot be considered nullified there.
Case one: then a tahor person drank
"V'shasah tahor, tamei." A person who was tahor then drank from the hollow. He becomes tamei, for the same reason we have described: in drinking he deliberately removed water from the pit, and we must be concerned that he ingested the tamei droplet along with the rest. This is by force of the rabbinic decree that one who drinks a revi'is of tamei liquids becomes tamei in his body and is disqualified from eating terumah.
Case two: then water was drawn with a tahor vessel
"Milah bichli tamei, u'milah bichli tahor, tamei." In this case nobody drank; rather, a second drawing was done, this time with a k'li that was tahor. We are required to treat that vessel as having become tamei, on the very same grounds explained already: the tamei k'li left behind a drop in the pocket, and when the tahor k'li lifts that water out, the tumah passes to it.
Case three: then a loaf of terumah fell in
"Milah bichli tamei, v'nafal kikar shel terumah." A tamei k'li was used to draw, and subsequently a kikar of terumah tumbled down into the depression.
"Im hediach, tamei." If he deliberately rinsed the loaf off in that water, he has thereby uprooted the water of the pit, and at that moment all of the water becomes tamei and renders the terumah tamei as well. Until such an act, nothing became tamei, because the loaf merely fell in; he had not uprooted any of the water. So long as the water remains connected to the ground, it is not susceptible to tumah.
"V'im lo hediach, tahor." If he did not rinse the loaf but simply lifted it out without any particular intent regarding the water, the loaf remains tahor. It does not receive tumah, because the water that came up with it was never deliberately removed from the ground.
So the lesson of this Mishnah is that the source of the tumah is interchangeable, a tamei drinker or a tamei vessel, while the determining factor stays constant: mei geva'im are immune to tumah as long as they rest in the earth, and it is the deliberate removal of the water that unlocks the tumah of the droplet hiding within it.