Makhshirim, Chapter Four, Mishnah One
1) If one bent down to drink, the water which came up on his mouth or on his moustache comes under the law of if water be put;
a) But [what came up] on his nose or on his head or on his beard does not come under the law of if water be put.
2) If one drew water with a jar, the water which came up on its outside, or on the rope which was wound round its neck, or on the rope which was needed for its use, comes under the law of if water be put.
a) How much rope is needed for its use? Rabbi Shimon ben Eleazar says: a handbreadth.
3) If he put the jar under the rain-pipe, it does not come under the law of if water be put.
Section one: When one drinks water from a river there is no way to avoid getting one’s mouth or mustache wet. Therefore, the water that comes up on his mouth or mustache is, in a sense, something he wanted, because without it, he couldn’t drink. It will therefore cause susceptibility. But he didn’t need to get his nose, head or beard wet. Therefore, the water that comes up with these parts of the body does not cause susceptibility.
Section two: When he lets the jar down into the cistern to bring up water, these parts of the jar/rope will certainly get wet. Therefore, the water on them will cause susceptibility. But he really only needs enough rope to lower the jug down into the cistern. Since he could lean down and fill the jug and only have a small bit of rope get wet, he doesn’t really need the rest to become wet. Therefore, the water on the rest of the rope does not cause susceptibility.
Section three: In this case, he wants the water to fall into the bucket and if he were to place it correctly there would be no rain on the outside the bucket. Therefore, any water on the outside of the jar does not cause susceptibility.