Machshirin, Chapter 4, Mishnah 6. This mishnah is a little intricate, but the principle behind it is one of the foundations of the whole masechta: produce becomes susceptible to tumah only when it has been wet by water that came upon it beratzon, and even then only when that water is detached water. Water that is still joined to the ground, such as the water of a mikveh or a spring, is not hechsher lekabel tumah at all. The psukim teach us this, and our mishnah shows how far the rule reaches: dry produce can sit inside a tamei vessel, be rinsed in connected water, and still emerge completely tahor.
The case: "Kupah shehi melei'ah turmosin" - a basket packed with turmosin, a bean that must be steeped and cooked in water so that its bitterness drains away and its taste turns sweet. Then, "unesanah letoch mikveh": that basket gets placed down inside a mikveh. Who placed it there? Picture a man who carries the status of an av hatumah and is holding the basket in his hands. By handling it he has turned the basket itself into a rishon letumah. Beans and all, it is set into the mikveh, and now, "moshit yado venotel turmos mitochah vehein tehorin," he puts his hand down into the water and lifts out a few of the beans.
And the ruling is that those beans come up tahor. Follow exactly where his hand rests: it is on the basket, never on the beans, so the only possible route of tumah to the beans would be from the basket, which carries the status of rishon letumah. Yet food is immune to tumah until hechsher has first made it receptive, and the sole liquid on these beans is mikveh water, which has never left the ground it belongs to. Water in that state has no power to be machshir. The result: the beans sit inside a vessel that really is a rishon letumah, and nothing whatsoever has made them capable of receiving it, so their taharah is intact.
Now the situation changes: "He'elan min hamayim, hanogei'a bakupah teme'in." Once the beans are lifted out of the mikveh, the moisture clinging to them is no longer part of a body of water joined to the ground; it is detached liquid, and detached liquid is hechsher lekabel tumah. From this moment the beans are susceptible. Therefore those beans that are actually in contact with the basket become tamei: the basket is a rishon, so what it touches becomes a sheini letumah.
"Ushe'ar kol haturmosin tehorin." The rest of the beans, the ones that are not resting against the basket but only against their fellow beans, remain tahor. The beans that did become tamei are only a sheini letumah, and in chullin there is no such status as a shlishi letumah. So even though these beans are touching something that is genuinely tamei, the chain stops there and they are unaffected.
The mishnah adds a second illustration of the same idea. "Tzenon shebame'arah, nidah medichaso vehu tahor." A radish is lying in the water of a spring inside a cave, and the radish has not yet been made susceptible to tumah. A nidah, who has the status of an av hatumah and would ordinarily convey tumah to food she touches, may rinse that radish while it is in the water, and it stays entirely tahor. Once again, the water is attached to the ground, so it cannot serve as hechsher, and without hechsher her touch has nothing to take hold of.
"He'elaso kol shehu min hamayim, tamei." Should she draw the radish up out of the water by even the smallest amount, it turns tamei. The wetness on its surface has now parted from the spring and counts as detached liquid, and the way she is handling the radish demonstrates that both its washing and its removal are to her satisfaction. Water that arrives beratzon is precisely what makes food receptive, so at that instant the radish is susceptible; it is being held by a woman who is a nidah, and the tumah takes hold at once.
The two halves of the mishnah teach one lesson from two directions. As long as the water remains part of the earth's own water, the strictest source of tumah in the world can be right there and the food is untouched. The instant that water is separated from its source and it is water that the owner is happy to have, the door to tumah swings open.