Makhshirin, Chapter Four, Mishnah Four
1) A jar into which a leak [from a roof] fell:
a) Bet Shammai say: it should be broken.
b) But Bet Hillel say: it may be emptied out.
2) But they agree that one may put out his hand and take produce from it and they are insusceptible to uncleanness.
Section one: The roof leaks into a jar full of produce. Clearly this was not something that the owner would have wanted. Thus, at this point the water has not made the produce susceptible to impurity. The problem is how can he get the water out without then making the produce susceptible? According to Bet Shammai he really can’t. Because once he tilts it over to the side the water will go from one side to the other and since this is something he wants, the produce will now be susceptible. The only way to get all the produce out without it becoming susceptible is to break the jar.
Bet Hillel says that as long as the water is in the jar, it is not something he wants and it won’t make the produce susceptible.
Section two: However, both houses agree that as long as he doesn’t shift the water around to pour it out, the produce will not be susceptible. He can reach his hand in and take out produce and it will remain insusceptible.