Machshirin, Chapter 4, Mishnah 7. The subject of this masechta is hechsher: water makes produce susceptible to tumah only when it came upon the produce beratzon, with the owner's satisfaction. Our mishnah presents a case where a man's own intention, and nothing else, decides whether the fruit ends up susceptible or not, and along the way it teaches us how a person's hands regain their taharah.
First, a word about tum'as yadayim. By Torah law, a person's hands become tamei only if he touches an av hatumah. The Chachamim, however, decreed a tumah on the hands themselves: if one's hands come into contact with a rishon letumah, the hands are treated as tamei, and if they then touch food, that food becomes a sheini letumah. It is a man in this rabbinic state whom the mishnah is describing.
The case: "Peiros shenaflu letoch amas hamayim, pashat mi shehayu yadav teme'os venatlan, yadav tehoros vehapeiros tehorin." Produce fell by itself into a water channel. A man whose hands were tamei reached into the water and picked the fruit up. The mishnah rules two things at once: his hands are now tahor, and the fruit is tahor as well.
Why are his hands tahor? Because immersing them in water is precisely how hands are purified from this rabbinic tumah, and here they were in the water. It makes no difference that purification was not on his mind at all and that he only wanted to retrieve his fruit; the immersion works regardless.
The Rav adds an important qualification. This taharah without intention serves him only for handling chullin. If he wishes to handle terumah or ma'asros, that is a different matter: there we require that the immersion be done with kavanah. It is specifically for the rabbinic tumas yadayim in relation to chullin that the water purifies even when he merely stuck his hand in to fish out the fruit.
And why is the fruit still tahor? Because it never became susceptible. The produce fell into the channel entirely against his wishes; there was no ratzon whatsoever that this water come upon it. Water that arrives without the owner's satisfaction is no hechsher, so even though his hands were tamei when he lifted the fruit out, there was nothing there for the tumah to attach itself to.
The final clause reverses the outcome: "Ve'im chishev sheyidachu yadav, yadav tehoros vehapeiros bechi yutan." Suppose that as he put his hand in, he was glad to have his hands washed clean by that water. His hands, of course, come out tahor. The fruit, though, has now entered the category of bechi yutan, ready to receive tumah. The difference lies in his attitude toward the water: he made use of it on purpose, for a benefit of his own, and so when he draws the dripping produce out of the channel, that last action is performed with his approval. Because the episode ended to his satisfaction, the whole wetting is credited to his ratzon, and the produce stands hechsher lekabel tumah.
Two men perform an identical action, reaching into a channel of water to pick up fallen fruit. One thinks nothing of the water, and both his hands and his fruit come out tahor. The other quietly welcomes the water for his hands, and his fruit has become susceptible to tumah. The mishnah could hardly make the point more sharply: in the laws of hechsher, everything turns on what a person wants.