Machshirin, Chapter 3, Mishnah 5. The subject of this masechta is hechsher lekabel tumah. Produce by itself cannot contract tumah; it must first be moistened by one of the seven mashkim, and even then the moistening counts only when it came about bechi yutan, with the owner's approval. Our mishnah presents another series of scenarios drawn from everyday farm work, and in each one the discussion turns on two points: is the wetness before us genuinely a liquid, and did the person want it there?
Softening produce in mud
"Hamatinim batit hanaguv." A person places produce into mud that is fairly dry, in order to draw some moisture out of it and soften the produce.
"Rebbi Shimon omer": for Rebbi Shimon the decisive factor is the wetness of the mud itself. Should the mud contain "mashkeh tofe'ach", moisture sufficient that a hand touching it comes away wet, then the produce set into it has undergone hechsher lekabel tumah.
"Ve'im lav, einan bechi yutan." If the mud is not wet to that degree, and the produce merely picks up a bit of dampness from lying buried in it, that is not bechi yutan. Moisture of that kind does not have the status of a liquid at all, and it cannot bring about hechsher.
Sprinkling the granary floor
Sprinkling the granary floor
"Hamarbits es gorno": a man wets down the floor of his granary so that the dust will not fly about. If he later spreads wheat over that same spot, the mishnah tells us "eino chosheish": he has no reason to suspect that leftover water reached his kernels. Granary ground is bone dry and drinks in water instantly, so we assume the surface had dried long before the grain was brought in, and no question of hechsher arises.
Grasses gathered with the dew on them
Grasses gathered with the dew on them
"Hamelakeit asavim keshetal aleihem": the custom was to bundle harvested wheat in blades of grass, "lehatis bahem chitim", because the dampness held inside the stalks helped keep the grain preserved. Now, plant sap does not appear among the shiv'ah mashkim, so on its own it cannot make wheat susceptible to tumah. Dew, tal, does belong to that list, and dew is fully capable of producing hechsher. Hence the mishnah's ruling: "einan bechi yutan", unless "niskaven lekach, harei zeh bechi yutan".
So picture a person who goes out in the morning and gathers his grasses while the dew is still on them. If the dew was of no interest to him, and he simply gathered in the morning because that was when it suited him, wanting nothing more than the moisture inside the grass itself, the dew is not bechi yutan and the wheat is unaffected. But if he specifically had the dew in mind, gathering the grasses wet so that the dew would keep his wheat moist, then he was working with one of the seven liquids by design, and the wheat has received hechsher lekabel tumah.
Rain on the way to the mill
Rain on the way to the mill
"Hamolich chitim lit-chon": a man is carrying his wheat to be ground, "veyardu aleihen geshamim", and rain starts coming down on the load. "Im samach": if the rainfall pleased him, the wetting is treated as bechi yutan, and the wheat is now fit to receive tumah.
Why would he be glad? The Rav explains that the kernels normally had to be soaked in water before grinding, and the rain now spares him that step. His pleasure is therefore genuine, and it attaches his will to the rainwater.
"Rebbi Yehuda omer": Rebbi Yehuda challenges gladness as the measuring rod here. No one hauling wheat toward the mill could fail to welcome such a rain, "ee efshar shelo lismo'ach", and an emotion that every person would feel discloses nothing about the will of this particular man. The test Rebbi Yehuda offers instead is "ela im amad": only where the fellow came to a halt and allowed the shower to soak his load do we treat the water as bechi yutan. Standing there like that proves beyond doubt that he wanted the wetting, and the wheat has then received hechsher lekabel tumah.
With this the mishnah concludes, and the mishnayos that follow will develop the question of intention further. What emerges here is a clear progression: first the mishnah asks whether the moisture even qualifies as a liquid (the mud, the granary floor), then whether the liquid is one of the seven (grass sap as against dew), and finally how we read a person's will when nature acts on its own.