Makhshirin, Chapter Four, Mishnah Two
1) If rain came down upon a person, even if he was unclean with a father of impurity, it does not come under the law of if water be put;
2) But if he shook it off, it does come under the law of if water be put.
3) If one stood under a rain-pipe to cool himself or to wash himself, [the water falling on him] is unclean if he is unclean;
a) If he is clean, it [only] comes under the law of if water be put.
Section one: The assumption the Mishnah makes is that a person does not want water to come down on his head. Therefore, the rainwater that falls on him remains pure, even if he is totally impure by being a “father of impurity” (for instance, he came into contact with a dead body).
Section two: However, once he shakes the water off his body, it now causes produce to be susceptible to impurity because it has been intentionally removed from its “source,” which in this case, is his body.
Section three: In this case he intentionally wanted the water to fall on him. Therefore, if he is unclean, the water becomes unclean. It also causes produce to be susceptible to impurity (unclean water always causes produce to be susceptible, but it doesn’t transmit impurity to non-sacred food).
If he is clean then the water is not yet unclean in and of itself. However, because he wanted it to come onto his body, it does make produce susceptible.
Again, we see here an excellent example of how intent dictates purity/impurity. If he intends to get wet, the water is impure or causes susceptibility.