Makhshirin, Chapter Two, Mishnah Four
1) If one flattened out his roof or washed his garment and rain came down upon it:
2) If the greater part consisted of the unclean water, it is unclean;
a) If the greater part consisted of the clean water, it is clean;
b) But if there was half of each, it is unclean.
3) Rabbi Judah says: if the dripping increased, [it is clean].
Section one: There are two situations in this mishnah. The first is that one is flattening out his roof and he pours out waste water at on the completed roof to smooth it out. The second is laundering a garment. In both of these cases the water being used is impure. This water is then mixed in with rain water and the question is whether the water is impure such that when it falls on food it would defile it.
Section two: As in the previous cases, the status of the water follows the majority. If it is half and half, then we rule strictly and it is unclean.
Section three: If the dripping from the roof or the laundered garment increased after the rain flow, then we can assume that the rain water was greater than the impure water from flattening the roof or laundering the garment. In this case, any water that subsequently comes from the roof or garment will be considered pure.