Makhshirin, Chapter Four, Mishnah Six
1) A basket full of lupines placed in a mikveh, one may put out his hand and take lupines from it and they remain clean.
2) But if he lifted them out of the water, those that touch the basket are unclean, but the rest of the lupines are clean.
3) If there was a radish in a cavern, a niddah may rinse it and leave it clean.
a) But if she lifted it, however little, out of the water, it becomes unclean.
Section one: In this case the basket was not put into the mikveh in order to rinse off the lupines. As long as the water in the mikveh remains connected to the mikveh it does not cause produce to become susceptible. And when he removes the lupines (a type of bean) he didn’t want the water to come out with them (he didn’t intend to rinse them off) so that water also does not cause susceptibility.
Section two: However, if he lifts the basket out of the water then the water on the sides of the basket does cause impurity because this water is something he would have wantedit cleans the sides of the basket. Therefore, any lupines that touch the sides of the basket are susceptible. The rest of the lupines are still not susceptible because the water that touched them is not something he wanted.
Section three: The water in the cavern is still in its source. Therefore it does not cause susceptibility. A niddah could put the radish in the water and it would remain clean. However, as soon as she removes it, even a little bit, the water causes the radish to be susceptible to impurity because the water has been removed from it source, and this was something the woman would have wanted (she put it in there to rinse it off).
Thus we see the difference in this mishnah between intentionally washing off, such as the case of the radish, and unintentional washing off. Only in the first case does the water that clings to the produce cause susceptibility.