Machshirin, Chapter 5, Mishnah 11. This mishnah closes the chapter with two separate topics. The first continues a theme developed just before it, that steam rising from a hot vessel can serve as a channel through which tumah passes upward. The second returns to the central subject of the entire tractate: which liquids count as hechsher, liquid that makes food susceptible to tumah, and which are simply unwanted moisture that a person never wanted at all.
The opening case: "Ha'isha shehayu yadeha tehoros," a woman with tahor hands, "umegisah bikdeirah temeiah," who is mixing the contents of a pot carrying tumah. "Im hezi'u yadeha, temeios": if the heat draws perspiration out onto her hands, those hands become tamei. The vapor climbing off the tamei pot carries its tumah upward to them, and the sweat that gathers there is the liquid through which it is transmitted. All of this rests on the rabbinic enactment of tumas yadayim, which is precisely why nothing beyond the hands is affected. She herself remains tahor, since liquid does not render a person tamei; only her hands take on the tumah.
The mishnah then flips the situation: "Hayu yadeha temeios," her hands already carried tumah, "umegisah bikdeirah tehorah," while the pot she is mixing is tahor. "Im hezi'u yadeha, hakdeirah temeiah": once perspiration appears on those tamei hands, the pot itself becomes tamei. The moisture forming on tamei hands is tamei moisture, and the vapor climbing off the pot forms a bridge linking her hands to what is inside it. Along that bridge the tumah travels down, and everything in the previously tahor pot is rendered tamei.
A stricter condition is set by Rabbi Yose: "im natafu." In his view the pot is only rendered tamei when the perspiration actually fell from her hands into it. Vapor by itself does not constitute a sufficient link; it must be plentiful enough to produce real liquid, and that liquid must land back in the food. Short of that, he does not declare the contents of the pot tamei.
A fresh subject now begins. "Hashokeil anavim bechaf mozneyim," someone places grapes on the pan of a balance to weigh them, and the weight of the fruit presses juice out so that it pools in the pan. Concerning that juice the mishnah rules "hayayin shebakaf tahor": it has no power to make anything susceptible to tumah. He was not manufacturing a drink, and leakage from his grapes is the last thing he wants, so this moisture emerged entirely against his wishes and does not qualify as hechsher. The exception is "ad sheye'areh lesoch hakli," until he tips it into a vessel. By collecting it he declares that it has value to him, and from that moment onward it does function as hechsher lekabel tumah.
"Harei zeh domeh," says the mishnah, drawing a parallel "lesalei zeisim va'anavim mishehein menatfim," to baskets holding grapes or olives from which liquid seeps out. In that case too, the wine or oil escaping from fruit stored in the basket is moisture the owner never sought, and it leaves nothing susceptible to tumah. And there as well, should he gather up what has dripped out, he has shown that the liquid is wanted, and at that point it becomes hechsher lekabel tumah.
So the chapter ends by binding its two halves together: liquid can carry tumah upward on a column of steam from a tamei pot to tahor hands, or downward from tamei hands into a tahor pot, but liquid that a person never wanted in the first place, juice pressed out on a scale pan or dripping through a basket, has no power to make anything susceptible at all, unless and until he decides to gather it.