Machshirin, Chapter 5, Mishnah 3. This chapter examines when water that reaches produce counts as having come with a person's consent. Only water that arrives beratzon is bechi yutan, water able to serve as hechsher and make food susceptible to tumah. Our mishnah presents a case where the water arrived entirely against the owner's will, and then the owner did something with it.
The Leaking Roof
The Mishnah teaches: "Peiros sheyarad hadelef letochan, uvelalan sheyivshu" - produce into which a leak dripped, and he mixed it so that it would dry. A person has a quantity of produce stored, let us say in a barrel, and water leaked in from the roof above. The leak wet only the upper layer, wherever the drops fell, while the produce lower down stayed dry. He certainly never wanted this water; at the moment it dripped in there was no question of bechi yutan at all, since the water came down against his will.
He then stirred the produce, mixing the wet with the dry. His purpose was to speed up the drying: instead of the moisture sitting concentrated in one place, it is now spread thinly through the whole mass and will evaporate faster. The question is whether that act of mixing changes the status of the water.
The Dispute
"Rabbi Shimon omer bechi yutan" - according to Rabbi Shimon the water now qualifies as bechi yutan. His position parallels other rulings we find: one who shakes a tree so that the moisture clinging to it should fall off has performed achshavah, an act that treats the water as something he cares about and thereby lends it halachic weight. The same applies in our case. Since the water is what prompted him to handle the produce, he has taken notice of it and granted it chashivus, and it therefore functions as hechsher and renders the produce fit to become tamei.
"Vachachamim omrim einan bechi yutan" - the Chachamim hold that it is not bechi yutan. Their reasoning is that he does not want this water in the slightest. The entire reason he mixed the produce was to make the moisture evaporate more quickly, that is, to be rid of it. Acting in order to remove water is not an act of making it significant; there is no machshiv here at all. Consequently the water remains as it was when it dripped in, and it cannot serve as hechsher to receive tumah.
The dispute thus turns on how to read an act done because of unwanted water: Rabbi Shimon sees any deliberate handling prompted by the water as conferring importance on it, while the Chachamim look at what the person actually wants, and one who is working to dry his produce plainly wants no water there at all.