Machshirin, Chapter 5, Mishnah 5. Masechta Machshirin rests on the Torah's condition "ki yutan mayim al zera": water prepares food to receive tumah only when it has come onto it with a person's consent, beratzon. Water that is still attached to the ground is never machshir; the entire discussion opens only once the water has been detached. Our mishnah tests that principle with a person poking around in a cistern to find out what is inside it.
The mishnah's first ruling concerns a man who has no idea whether the cistern below him holds any water. He stretches down into it, either "yado" (his hand), or "raglo" (his foot), or "kaneh" (a reed), lowering it into "habor", the pit, and his purpose is "leida", to know, "im yesh bo mayim" - whether there is water down there at all. Concerning the moisture that comes back up on his hand, his foot or the reed, the mishnah rules "eino bechi yutan": such water carries no power to make food susceptible to tumah.
Why not? Because he has no use whatsoever for the water once it is out. His entire interest was in the information, and the information is delivered the instant he feels wetness down below. From that moment on the water on his arm serves him not at all. The water inside the cistern, still joined to the ground, was never a candidate for being machshir to begin with; and the drops that were detached and carried out are wanted by nobody. With no consent, there is no hechsher lekabel tumah.
The mishnah continues: "Leida kama mayim yesh bo, harei zeh bechi yutan." If he lowered his hand, his foot or the reed not to learn whether there is water but to learn how much water there is, the water is bechi yutan. Here the moment of contact tells him nothing. He has to draw the reed out and look at it, or pull his foot out and see how far up the wet line reaches, and he can only do that with water still on it. In other words, he needs that water after it has been detached, and needing it is precisely what makes it wanted. Since he is satisfied to have it there, it counts as beratzon and it is machshir.
A parallel case closes the mishnah. Someone drops a rock down the shaft - "zarak even" into "habor" - and his purpose, again, is "leida", to know, "im yesh bo mayim": he is listening for the sound of a splash. And indeed there is water, and the stone sends up a spray. Of those airborne droplets the mishnah says "hanitazin einan bechi yutan", and of the film left clinging to the rock it says "veshebe'even tehorin". When the mishnah calls them tahor, the intent is that they have no capacity to be machshir: they are outside the category of bechi yutan, and nothing they touch becomes able to receive tumah through them.
The reasoning is the same as in the opening clause. What he wanted was the answer to a question, and the splash itself already answered it. The water lying in the cistern is attached to the ground and cannot be machshir in any case, and once he no longer has the slightest interest in the drops that left it, those drops were never detached beratzon. Both the flying spray and the film of water on the stone remain outside the category of bechi yutan.