Mikvaos, Chapter 1, Mishnah 3. Our chapter is working its way through the various grades of water and the differing degrees of taharah they can provide. This mishnah continues discussing may geva'im: rainwater that has fallen naturally and collected in a hollow, a puddle or a pit in the ground, in a quantity of less than forty se'ah. Such water is not a valid mikveh, yet it is not simply ordinary water either. It carries certain properties of taharah, and the mishnah spells out how those properties behave when tumah is introduced.
The principle underlying the cases here is this: when tamei water falls into the collected rainwater, the offending drop is swallowed up in the larger body of water and is neutralized while it remains there. The moment water is drawn out of the pit leratzon, deliberately and with intent, the tumah reasserts itself, since we must reckon with the possibility that the original drop was part of what was removed.
A tahor person who drinks
The mishnah begins: "Naflu mayim teme'im", tamei waters fell into this puddle, "veshosah tahor", and afterward a person who was tahor drank from it. He is tamei. Because he drew the water into his mouth willingly, the water he took was uprooted from the pit leratzon, and we are concerned that the original tamei droplet was mixed into what he swallowed. Having ingested tamei water, he himself becomes tamei.
Filling a vessel
The second case in the mishnah: "Naflu mayim teme'im", tamei waters landed in the geva, "umilah bekheli tahor", and a person then took a keli that had been tahor and filled it from that collected rainwater. The vessel has become tamei. Here too the filling was a deliberate act, so we must allow for the possibility that the unclean droplet rose together with everything else that entered the keli, conveying tumah to it.
A loaf of terumah
The third case: "Naflu mayim teme'im", tamei waters landed in the hollow, "venafal kikkar shel terumah", and then a round loaf of bread that had the status of terumah tumbled in as well. What becomes of it depends entirely on how its owner went about recovering it. "Im hidiah", if he rinsed the loaf, dashing some of the water in the hollow over it, then he has purposely displaced water from where it had settled; that water regains its tumah and passes it along, and the loaf is tamei. "Ve'im lo hidiah", but if he did no rinsing and simply raised the bread and carried it off, he never intended to take any water at all, only to salvage his food. The loaf stays tahor.
The view of Rebbi Shimon
To this the mishnah records a dissent: "Rebbi Shimon omer, bein hidiah bein shelo hidiah, tamei." Whether he rinsed the loaf or not, it is tamei.
The Rosh understands Rebbi Shimon's concern in a narrow way. Even when the loaf is simply lifted out, water clings to it as it emerges, and we must be apprehensive that the drop of originally tamei water came up along with the loaf and conveyed tumah to it.
The Beiur HaGra, in his commentary on the Tosefta, reads Rebbi Shimon far more broadly. In his view, Rebbi Shimon is not merely arguing about the loaf; he rejects the entire premise of the Tanna Kamma's rulings until now. The Tanna Kamma holds that so long as the offending droplet sits within the body of water it is nullified and batel. Rebbi Shimon maintains that there is no such nullification. The tamei water undergoes no change of status simply by virtue of having landed in the geva; it remains tamei where it lies. Consequently, the instant the loaf fell into the water it contracted tumah, and the question of whether he rinsed it afterward is beside the point.
The three cases of our mishnah thus turn on a single hinge: intentional removal of water from the pit. According to the Tanna Kamma, drinking and drawing and rinsing all qualify, while merely retrieving a fallen object does not. According to Rebbi Shimon, as the Gra explains him, the tumah never went away in the first place.