Machshirin, Chapter 3, Mishnah 1. After a stretch of mishnayos that dealt with cases of doubt resolved by following the majority, the third chapter brings us back to the heart of the masechta: liquid that comes onto food and renders it susceptible to tumah. The Torah's condition is that the moisture arrive with the owner's approval, expressed in the phrase bechi yutan. When that condition is met, the food has received hechsher lekabel tumah, the preparation that allows it to become tamei.
The mishnah opens with a loaded sack: "Saq shehu malei peiros unesano" - a sack filled with produce, which its owner set down somewhere. Most of the commentators take the produce here to be grain, and wheat in particular. Where was it put down? Three locations are listed. "al gav hanahar" - alongside a river; "o al pi habor" - or at the opening of a cistern; "o al ma'alos hame'arah" - or on the stairway that leads down into a cave, which the Rav identifies as a cave with water inside it. In each of the three, the wheat pulled in moisture from the nearby water without ever being dipped into it; the dampness of the surrounding air was enough. And the halachah: "vesha'avu, kol shesha'avu bechi yutan" - whatever soaked up moisture in this manner falls under bechi yutan.
Two elements combine to produce this ruling. First, as the Rav explains, the owner is pleased with what happened: grain that takes in moisture swells and plumps out and presents better, so the dampness is genuinely desirable to him. Second, he is the one who placed the sack there. Since he set it down in a spot where the produce would inevitably draw in moisture, and the result is to his benefit, the water counts as having come with his consent. The grain has therefore received hechsher lekabel tumah.
Rebbi Yehuda draws a line within the sack. "Rebbi Yehuda omer" - and his words are: "kol shehu keneged hamayim bechi yutan, veshe'eino keneged hamayim eino bechi yutan." Whichever grain lies opposite the water is bechi yutan, while grain positioned away from it is excluded. His logic: only the kernels along the side of the sack turned toward the water took in their dampness from the water itself. Kernels packed deeper inside got wet from their neighbors, from grain that had already absorbed the moisture. Such wetness arrives second hand, one remove away from its origin, and it is not viewed as having come from the water beside which the owner rested his sack.
The Tanna Kamma does not accept this distinction, holding that the entire contents of the sack are rendered susceptible, and the halachah is not decided in accordance with Rebbi Yehuda.
So concludes the first mishnah of the chapter.