Machshirin, Chapter 3, Mishnah 4. Every mishnah in this masechta circles back to a single idea, hechsher lekabel tumah. Food that has stayed dry cannot become tamei at all; it must first meet water, or any of the other six liquids the chachamim listed, and even that contact only counts when it came about bechi yutan, in a way the owner wanted. Here the mishnah walks through several ordinary situations in which grain was found to be moist, and in each one the same question is asked: was there a person standing behind that moisture, or did it come from a source nobody ever arranged?
Sprinkling the floor of the house
"Hamarbits es beiso." A man wets down the floor of his house. The Rav explains that the floor here is exposed bedrock, and the owner sprinkles water over it so that the dust will not fly up. Later he spreads out his grain on that very floor: "nasan bo chitim venisnenu," and the kernels turn out to be damp.
"Im meichmas hamayim, bechi yutan." If the dampness on the wheat came from the water he himself sprinkled, that counts as wetting with his consent, and the wheat has received hechsher lekabel tumah.
"Im meichmas hasela, einam bechi yutan." But if the moisture came from the bedrock itself, the stone giving off its own dampness the way a surface perspires, no person put that liquid there. It is not bechi yutan, and the wheat has not become susceptible to tumah.
Wheat placed in a laundry trough
"Hamachabeis es kesuso ba'arivah." A man washes an article of clothing in a trough. When he is done he uses the same trough for grain, "nasan bo chitim venisnenu," and the kernels are found to be moist.
"Im meichmas hamayim, bechi yutan." If the wetness came from the wash water, it is bechi yutan. The water was brought into the trough by his own choice from the very start, so it carries his will with it; and he is pleased that his wheat has now become damp, since the moisture improves how the kernels look. The wheat is therefore made fit to receive tumah.
"Meichmas atzman, einam bechi yutan." If, however, the wheat became wet from itself, because the kernels were pressed together tightly in the trough and gave off their own inner moisture, that is nobody's doing. Such wetness is not bechi yutan.
Burying produce in sand
"Hamatinim bachol, harei zeh bechi yutan." It was a common practice to bury produce in sand so that the natural dampness of the sand would soften and moisten it. Since sand as a rule holds moisture, and since that moisture is precisely the result a person is counting on when he buries his produce in it, the wetting is considered bechi yutan and the produce receives hechsher.
The people of Machoz
"Ma'aseh be'anshei Machoz." The mishnah reports an incident involving the townspeople of Machoz, "shehayu matinim bachol": it was their standing custom to soften grain by laying it under sand. They went on doing this for years, taking for granted that the practice had no bearing whatsoever on the tumah status of what they grew.
"Amru lahem chachamim." The chachamim answered them: "im kach heyeisem osim" - if that has been your way of doing things, then "lo asisem tahor miyemeichem," not one tahor batch has ever left your hands in your entire lives. From the moment each load of grain was set down in the moist sand it had already acquired hechsher lekabel tumah, and the flour ground from it was destined to turn tamei. The custom was one they never should have taken on, for the simple reason that it exposed everything they produced to tumah.
This is where our mishnah closes. Its lesson is a consistent one: the halachah does not ask only whether produce is wet, but who is responsible for the wetness. Water a person sprinkled, water he poured into a trough, moisture he counted on drawing out of the sand, all of these come with his will attached. Dampness rising out of the bedrock, or moisture squeezed out of the kernels by their own crowding, belongs to nobody, and leaves the produce untouched.