Machshirin, Chapter 4, Mishnah 8. Until now the chapter has been occupied with the laws of hechsher, the wetting of produce that prepares it to receive tumah. Here the Mishnah turns to a related question in the laws of tumah and taharah: what happens to a vessel, and to the liquid inside it, when the vessel is sitting inside a mikveh and a tamei person reaches in?
The Mishnah's case: an earthenware pot, a kli cheres, is filled with water and then lowered into a mikveh, so that it rests there with mikveh water surrounding it on the outside and its own water held within. In the Mishnah's words, "Kedeirah shemileiah mayim", and "unesunah lesoch hamikveh". Then "ufashat av hatumah", a person carrying the status of a primary source of tumah, "es yado lesocha", reached into the pot. The verdict is "temeiah": the pot has become tamei.
Why the hand alone is enough
An earthenware vessel has a rule of its own. It does not need to be touched on its wall or its rim in order to contract tumah. It is enough that something tamei enters the space of its interior, its airspace, even without making contact with any part of the vessel. So the moment the av hatumah put his hand into the airspace of the kedeirah, the vessel itself was exposed to tumah.
The water inside stays tahor, the pot does not
Now the pot is standing in a mikveh, and that changes the picture for the water it contains. The Rav explains that the water inside the kedeirah does not become tamei, because it is in contact with the water of the mikveh. Since the mikveh water is fit to purify the water in the pot, that contact keeps the water inside from ever becoming tamei to begin with.
The pot itself, however, has no such protection. The mikveh water does not reach it, because the water sitting inside the kedeirah stands between the two and prevents the mikveh water from purifying the vessel. The result is exactly as the Mishnah rules: the water inside remains tahor, while the pot remains tamei.
A weaker grade of tumah
"Maga tumos, tehorah." Now change the case. The hand reaching in belongs to someone whose own tumah came to him by touching a source of tumah, so that an av hatumah stands one level above him; this grade is what we ordinarily call a rishon letumah. In such a case the ruling reverses and the pot remains tahor. The principle behind this is a broad one: tumah of the first degree has no power to pass on to a kli, and only a primary source of tumah can render a vessel tamei. This man's tumah is a step removed, so his hand entering the inner space of the earthenware pot leaves the vessel untouched.
When the pot contains a different liquid
"Ushear kol hamashkim, temein, she'ein hamayim metaheres es shear hamashkim." Suppose the pot was filled not with water but with some other liquid. Now both the liquid and the pot become tamei. The reason is given by the Mishnah itself: water does not purify other liquids.
The whole reason the water in the pot remained tahor in the first case was hashakah, the contact between it and the water of the mikveh. But hashakah works only when both sides are water. When the liquid in the pot is something else, the mikveh water does not purify it at all. That liquid can therefore become tamei, and liquids, unlike vessels, are made tamei even by a rishon letumah and not only by an av hatumah. Once the liquid inside the pot has become tamei, it in turn conveys tumah to the pot that holds it. Hence the Mishnah's ruling that in such a case the liquid inside and the earthenware pot are both tamei.