Machshirin, Chapter 4, Mishnah 5. This chapter revolves around the rule that liquid renders food susceptible to tumah only when it came with a person's consent, as the pasuk teaches, "vechi yutan mayim," if water be put upon the seed. Water that a person wanted is "bekiyutan" and brings hechsher lekabel tumah; water that arrived on its own does not. Our Mishnah takes a single vessel sitting under a dripping leak and asks a series of questions: does it matter whether the vessel was deliberately placed there? Does it matter what he later does with the water? And what about the water that splashes out or spills over the rim rather than staying inside?
The bowl that happened to be there
"Areivah sheyarad hadelef letochah, hanitzanin vehatzofin einan bekiyutan." A bowl was lying somewhere, open side up, and a leak dripped into it. "Hanitzanin" are the drops that spatter out, and "hatzofin" are the waters that overflow the rim. These are not bekiyutan, and neither is the water resting inside the bowl. The reason is simple: he never set the bowl there to catch anything. It happened to be there, the leak found it, and none of this water is water he wanted.
He picks it up to pour it out
"Netalah leshofchah": he now takes hold of the bowl and carries it off so as to empty it somewhere else. According to Beis Shammai, the water has by this act become bekiyutan. Their logic: a person who regarded the water as sheer nuisance would have tipped it over on the very spot where the bowl stood. Bothering to bring it elsewhere before disposing of it reveals a certain concern for it, and that measure of concern is enough for the water to give hechsher lekabel tumah.
Beis Hillel rule otherwise, and for them the water stays outside the category of "bekiyutan." He never positioned the bowl, and the dripping was never something he sought. Handling the vessel afterward cannot manufacture consent after the fact. Had he lifted it and repositioned it in order to catch the drops, that might well have qualified; but carrying a bowl whose contents he plans to dump yields him nothing he benefits from, and without benefit there is no ratzon.
He placed the bowl there on purpose
"Hinichah sheyeired hadelef letochah": this time he positioned the vessel beneath the drip on purpose, catching the water as it fell, perhaps so that the floor underneath would stay dry. Everyone concurs that whatever gathers within such a bowl is bekiyutan, since he wanted the water accumulated there and had a use in mind. The argument concerns only "hanitzanin vehatzofin," the drops that spatter beyond the vessel and the water that runs over its edge.
According to Beis Shammai, once the collecting of this water was intended from the start, everything that leaves the vessel inherits the identical standing. True, he derives no pleasure whatsoever from what spills out and would have preferred that it remain inside; even so, the consent given at the outset covers the entire quantity and has not lapsed.
Beis Hillel limit that consent to its actual object, the water that stayed in the vessel. Drops that flew off or ran over the lip were never wanted, and concerning them the ruling is "einan bekiyutan."
Where both agree
"Netalah leshofchah, eilu va'eilu modim shehein bekiyutan." Should he now take up that same bowl and carry it away in order to empty it, Beis Shammai and Beis Hillel are of one mind that the water is bekiyutan. The reference here is to whatever had accumulated within the vessel. Emptying it does indeed prove that he has no further use for it; nevertheless, it was gathered there with his complete approval from the beginning, and that earlier desire is not undone. The water keeps its standing as biratzon, it is bekiyutan, and it confers hechsher lekabel tumah.
Wet hands and wet feet
"Hamatbil es hakeilim vehamechabes kesuso bame'arah, vehamayim ha'oleh beyadav bekiyutan." Picture someone dipping utensils, or scrubbing his cloak, in the water of a cave. Whatever water comes up onto his hands carries the status of bekiyutan. Wetting the hands belongs to the task itself: nobody immerses a keili or launders a garment while keeping his hands dry. Since it is inseparable from what he set out to accomplish, such water counts as biratzon and can render food susceptible to tumah.
As for whatever clings to his feet, the Mishnah rules "beraglav einan bekiyutan." Wet feet are neither desired by him nor demanded by the work, so this water lacks the power to bring hechsher.
Rabbi Elazar qualifies the ruling: "im i efshar lo sheyeired," if there is no way for him to go down into the cave "ela im kein," unless, "nitanfu raglav," his feet are wetted in the process, then the water on his feet, too, falls under bekiyutan. Whenever soaked feet are an unavoidable byproduct of the very activity he chose to do, they are swept up in his consent, and that water likewise renders food fit to receive tumah.
Throughout this Mishnah one principle keeps sharpening itself: consent is measured not only by what a person wants, but by how tightly the result is bound to what he wanted. A bowl he never placed, water he means only to discard, feet he never intended to wet: each of these is weighed against the original act of will, and the halachah turns on how far that will reaches.