Machshirin, Chapter 3, Mishnah 8. The masechta as a whole asks when water that comes onto produce counts as having arrived biratzon, with the owner's approval, so that it renders the produce hechsher lekabel tumah, susceptible to receiving tumah. Water that lands by itself accomplishes nothing; water that a person wanted accomplishes everything. Our Mishnah works out that principle in two settings of ordinary farm life: soaking wooden implements, and watering an animal.
Soaking wheels and cattle gear
Case one deals with wooden farm equipment. "Hamorid es hagalgalim": a man takes the wheels of his wagon, and with them "klei habakar," the wooden gear used in working oxen, and sets them down "lamayim," into water, "bish'as hakadim," in the season of the parching east wind, "bishvil sheyichotzu," so that they should tighten up, "harei zeh bechi yutan." Why would a farmer bother? Because blistering dry heat shrinks timber and opens fissures in it. The remedy of that era was immersion: waterlogged wood swells and the gaps close again. Since the man himself placed the wood into the water, that water carries the status of biratzon, and should it later touch food it gives hechsher.
On the face of it this is straightforward. Even water that arrives on its own can sometimes be biratzon, as the earlier Mishnah taught, so water that a man intentionally submerges his utensils in is certainly wanted. What, then, is the novelty here? Some explain that the Mishnah is teaching about the leftover water clinging to the rest of the wood. Only the cracked areas needed soaking, yet the entire implement comes up dripping, including places where there was no split at all. Even that residual water, water he never specifically needed, is bechi yutan, and if it touches produce it is a hechsher. Once a person immerses an object for a purpose of his own, all the water that comes up with it is counted as wanted.
Bringing an animal to drink
The second case: "Hamorid beheimah lishtos," one who takes his animal down to drink. The ruling is that "hamayim ha'olim befiha," the water that comes up on its muzzle, is "bechi yutan," whereas regarding "ragleha," its feet, the water there is not. What lies behind the distinction? Nobody walks a beast to a spring in order to moisten its mouth, yet every owner takes for granted that the animal will lift its head dripping. Water whose arrival is a foregone conclusion of the very act a person chose to perform is water that came with his consent, and if it reaches produce it gives hechsher.
Not so the water on its feet. There a person would honestly rather the animal stay dry, because a wet animal picks up mud. That water is unwanted, so it is not bechi yutan.
"Im chishev sheyudachu ragleha": if the owner took the beast down precisely so that its hooves would be rinsed off, then the water found "beragleha," on its legs, also carries the status of "bechi yutan." Wetting the legs is here his very objective, and once more, whatever extra water tags along is swept into the same ruling.
"Bish'as hayachaf vehadayish, le'olam tamei." In the period when animals suffer from tender, worn hooves, and in the period of threshing, this water invariably confers hechsher. A word about the term "tamei" as used here: the Mishnah does not mean that the water itself contracted tumah, but that the water is treated as wanted and therefore makes produce fit to receive tumah. Why should these seasons differ? Beasts driven over long distances grow footsore, and that condition tends to fall out around threshing time. In such a period an owner is pleased when the water reaches the legs, since soothing those sore hooves is part of his reason for taking the animal down to the water at all. So the Ra"v explains, and he offers a second reading as well: at threshing one wants the animal's hooves free of dirt so that it not foul the grain it treads. On either reading, the descent to the water is undertaken with the wetting in view, so the water came biratzon and gives hechsher.
Action without intent
"Horidah cheresh, shoteh, vekatan": now suppose the animal was taken to the water by a deaf-mute, by someone mentally incompetent, or by a child. The Mishnah rules that "af al pi shechishev sheyudachu ragleha," even though he formed the intention that the hooves be rinsed, the water on the legs does not count as wanted, "sheyesh lahem ma'aseh," because these three possess deed, "ve'ein lahem machshavah," while intent they do not possess.
The point is this: a cheresh, shoteh or katan lacks halachically recognized machshavah. Where the water's status depends purely on intention, their intention counts for nothing, so the water that comes onto the animal's legs while they simply lead it to drink cannot be a hechsher. But where there is an actual deed, their deed does count. If one of them took hold of the animal's feet and washed them, that is a ma'aseh, an active act of wetting, and such water is biratzon and is bechi yutan. The dividing line is exactly there: thought alone from these three achieves nothing; an act performed by them achieves what an act achieves.
So this Mishnah leaves us with a clear yardstick. Water is wanted when a person purposely brings it about, and also when it is the inevitable companion of what he purposely brought about, but only where the halachah credits him with intent in the first place.