Machshirin, Chapter 1, Mishnah 2. This masechta, whose name means "preparations," deals with a single subject: how food becomes susceptible to tumah in the first place. Food is not automatically capable of contracting tumah. It must first receive a hechsher, a preparation, and that hechsher comes about through contact with liquid, one of the seven liquids that qualify. Once a food has been wetted, it can become tamei.
But there is a condition. The Torah writes "v'chi yutan mayim al zera," and the word is spelled without a vav, so that it can be read either as "yutan," the water was placed there by some outside cause, or as "yitein," that you placed it there yourself. Chazal derive from this that water which arrives on its own can indeed make food susceptible, but only if it arrives in a way that resembles your having placed it, meaning that the wetting was pleasing to you. Ratzon, the owner's satisfaction with the food becoming wet, is the gateway to hechsher throughout this masechta. Our Mishnah tests that condition against a case of a man shaking a tree.
Shaking to Dislodge Food or Tumah
The Mishnah begins: "Hamarit es ha'ilan l'hashir mimenu ochlin o es hatumah, einan b'chi yutan." A person shakes a tree in order to bring down food from it, perhaps fruit he wants to harvest, or in order to dislodge something tamei that is lodged in its branches, some source of tumah he wants out of the tree. As he shakes, water that had been sitting on the leaves is knocked loose and lands on his food.
The Mishnah rules that this water is not within "b'chi yutan." It is not treated as water whose falling pleased him, and it therefore does not make the food a hechsher l'kabel tumah. The reason follows directly from the principle: he never wanted that water to come down. His entire intention was to remove the fruit or to remove the tumah from the tree, and the water's descent was incidental to what he was after. Since hechsher requires that the wetting be desired, water shaken loose along the way while the man's mind was on something else does not render the food susceptible to tumah.
Shaking to Remove the Water Itself
The Mishnah now turns to a different case: "l'hashir mimenu mashkin." Here the man shakes the tree precisely in order to get the water out of it. He wants that water gone, perhaps because it is damaging something in the tree. Now the water's departure is exactly what he intended, and Beis Shammai and Beis Hillel divide over how far that intention reaches.
"Beis Shammai omrim: hayotz'in v'she'bo b'chi yutan." According to Beis Shammai, both the water that came out and the water that stayed behind are in the category of "b'chi yutan." The water he succeeded in shaking off will certainly make food susceptible to tumah if it lands on it. But so will the water still clinging to the tree, the water he did not manage to dislodge. If that remaining water later falls on food, the food becomes a hechsher l'kabel tumah. The Rav explains the logic: by turning his attention to this water and acting to remove it, the man conferred chashivus, significance, upon all of the water there, both what came down and what is still in the branches. Once the water as a whole has been given that standing, all of it belongs to "b'chi yutan."
"Beis Hillel omrim: hayotz'in b'chi yutan, v'she'bo eino b'chi yutan, mipnei shehu miskaven sheyeitzei mikulo." Beis Hillel agree about the water that came out. That water left the tree exactly as he wished, so it is within "b'chi yutan," and food it wets becomes capable of receiving tumah. But the water that remained on the tree, they hold, is not within "b'chi yutan." The reason is in his own intention: he wanted every last drop to leave the tree. Whatever water is still sitting there represents the part of his plan that failed, and he has no further interest in it. Water he no longer cares about is not water whose presence pleases him, so it does not fall under the pasuk of "b'chi yutan," and it does not make food a hechsher l'kabel tumah.
The Mishnah thus draws a fine and instructive line. The same water on the same tree can be inside or outside the category of "b'chi yutan" depending entirely on what the person shaking the branches had in mind, and, according to Beis Hillel, on whether his intention was actually fulfilled.